System and Method for End-to-End Beamforming

ABSTRACT

Methods and systems are described for providing end-to-end beamforming. For example, an end-to-end beamforming system include a relay satellite and a ground network to provide communications to user terminals located in user beam coverage areas. The ground network includes geographically distributed access nodes and a central processing system (CPS). Beamformers of the ground network generate forward uplink signals from appropriately weighted combinations of user data streams that, after relay by the satellite, produce forward downlink signals that combine to form forward user beams.

TECHNICAL FIELD

The disclosed systems, methods, and apparatuses relate to end-to-endbeamforming in a system using an end-to-end relay.

BACKGROUND

Wireless communication systems, such as satellite communication systems,provide a means by which data, including audio, video, and various othersorts of data, may be communicated from one location to another.Information originates at a first station, such as a first ground-basedstation, and is transmitted to a wireless relay, such as a communicationsatellite. Information received by the wireless relay is retransmittedto a second station, such as a second ground-based station. In somewireless relay communication systems, either the first or second station(or both) are mounted on a craft, such as an aircraft, watercraft, orlandcraft. Information may be transmitted in just one direction (e.g.,from a first ground-based station to a second ground-based station only)or may be transmitted in both directions (e.g., also from the secondground-based station to the first ground-based station).

In a wireless relay communication system in which the wireless relay isa satellite, the satellite may be a geostationary satellite, in whichcase the satellite's orbit is synchronized to the rotation of the Earth,keeping the coverage area of the satellite essentially stationary withrespect to the Earth. In other cases, the satellite is in an orbit aboutthe Earth that causes the coverage area of the satellite to move overthe surface of the Earth as the satellite traverses its orbital path.

The signals that are directed to or from a first station may be directedby using an antenna that is shaped to focus the signal into a narrowbeam. Such antennas typically have a paraboloid shaped reflector tofocus the beam.

In some cases, a beam may be formed electronically by adjusting the gainand phase (or time delay) of signals that are transmitted, received, orboth from several elements of a phased array antenna. By properlyselecting the relative phase and gain transmitted and/or received byeach element of a phased array antenna, the beam may be directed. Inmost cases, all of the energy being transmitted from a ground-basedstation is intended to be received by one wireless relay. Similarly,information received by the second station is typically received fromone wireless relay at a time. Therefore, it is typical that a transmitbeam that is formed to transmit information to the wireless relay(whether by use of electronic beamforming or by use of an antenna with ashaped reflector) is relatively narrow to allow as much of thetransmitted energy as possible to be directed to the wireless relay.Likewise, a receive beam that is formed to receive information from thewireless relay is typically narrow to gather energy from the directionof the wireless relay with minimal interference from other sources.

In many cases of interest, the signals that are transmitted from thewireless relay to the first and second stations are not directed to asingle station. Rather, the wireless relay is able to transmit signalsover a relatively large geographic area. For example, in one satellitecommunication system, a satellite may service the entire continentalUnited States. In such a case, the satellite is said to have a satellitecoverage area that includes the entire continental United States.Nonetheless, in order to increase the amount of data that may betransmitted through a satellite, the energy transmitted by the satelliteis focused into beams. The beams may be directed to geographic areas onthe Earth.

BRIEF DESCRIPTION OF THE FIGURES

The drawings are provided for purposes of illustration only and merelydepict examples. These drawings are provided to facilitate the reader'sunderstanding of the disclosed method and apparatus. They do not limitthe breadth, scope, or applicability of the claimed invention. Forclarity and ease of illustration, these drawings are not necessarilymade to scale.

FIG. 1 is an illustration of an example of a satellite communicationsystem.

FIG. 2 is a diagram showing an example pattern of beams that covers thecontinental United States.

FIG. 3 is an illustration of an example of the forward link of asatellite communication system in which the satellite has a phased arraymulti-feed per beam on-board beamforming capability.

FIG. 4 is an illustration of an example of the forward link of asatellite communication system having ground-based beamforming.

FIG. 5 is an illustration of an example end-to-end beamforming system.

FIG. 6 is an illustration of example signal paths for signals in thereturn direction.

FIG. 7 is an illustration of example signal paths in the returndirection from a user terminal.

FIG. 8 is a simplified illustration of an example end-to-end returnchannel matrix model.

FIG. 9 is an illustration of example signal paths in the forwarddirection.

FIG. 10 is an illustration of example signal paths in the forwarddirection to a user terminal located within a user beam coverage area.

FIG. 11 is a simplified illustration of an example end-to-end forwardchannel matrix model.

FIG. 12 is an illustration of an example end-to-end relay satellitesupporting forward and return data.

FIG. 13 is an illustration of an example of an uplink frequency rangebeing divided into two portions.

FIG. 14 is an illustration of an example end-to-end relay being timemultiplexed between forward data and return data.

FIG. 15 is a block diagram of components of an example end-to-end relayimplemented as a satellite.

FIG. 16 is a block diagram of an example transponder including a phaseshifter.

FIG. 17 is a graph of example signal strength patterns of severalantenna elements.

FIG. 18 is an illustration of example 3 dB signal strength contours forseveral antenna elements.

FIG. 19 is an illustration of example overlapping signal strengthpatterns of several antenna elements.

FIG. 20A-20E is an illustration of example overlapping 3 dB signalstrength contours for several antenna elements.

FIG. 21 is an illustration of an example enumeration of 16 antennaelements and their overlapping 3 dB signal strength contours.

FIG. 22 is a table showing example mappings of receive antenna elementsto transmit antenna elements through 16 transponders.

FIG. 23 is an illustration of a cross-section of a paraboloid antennareflector and an array of elements centered at the focal point of theparabola.

FIG. 24 is an illustration of a cross-section of a paraboloid antennareflector and an array of elements placed away from the focal point ofthe parabola.

FIG. 25 is an illustration of an example relay coverage area (shown withsingle cross-hatching) and the area (shown with double cross-hatching)defined by the points within the relay coverage area that are alsocontained within six antenna element coverage areas.

FIG. 26 is an illustration of an example relay antenna pattern in whichall of the points within a relay coverage area are also contained withinat least four antenna element coverage areas.

FIG. 27 is an illustration of an example distribution of access nodes(ANs) and user beam coverage areas.

FIG. 28 is an example graph of normalized forward and return linkcapacity as a function of the number of ANs deployed.

FIG. 29 is a block diagram of an example ground segment 502 for anend-to-end beamforming system.

FIG. 30 is a block diagram of an example forward/return beamformer.

FIG. 31 is a block diagram of an example forward beamformer comprisingmultiple return time-slice beamformers with time-domain de-multiplexingand multiplexing.

FIG. 32 is an illustration of a simplified example ground segmentshowing the operation of a forward time-slice beamformer.

FIG. 33 is a block diagram of an example return beamformer comprisingmultiple return time-slice beamformers with time-domain de-multiplexingand multiplexing.

FIG. 34 is an illustration of a simplified example ground segmentshowing the operation of a return beamformer employing time-domainmultiplexing.

FIG. 35 is a block diagram of an example multi-band forward/returnbeamformer that employs sub-band de-multiplexing and multiplexing.

FIG. 36 and FIG. 37 is an illustration of example timing alignment forthe forward link.

FIG. 38 is a block diagram of an example AN.

FIG. 39 is a block diagram of part of an example of an AN.

FIG. 40 is a block diagram of an example AN 515 in which multiplefrequency sub-bands are processed separately.

FIG. 41 is an illustration of an example end-to-end beamforming systemfor enabling distinct user-link and feeder-link coverage areas.

FIG. 42 is an illustration of an example model of signal paths forsignals carrying return data on the end-to-end return link.

FIG. 43 is an illustration of an example model of signal paths forsignals carrying forward data on the end-to-end forward link.

FIGS. 44A and 44B are an illustration of an example forward signal pathand return signal path, respectively.

FIGS. 45A, 45B, 45C, 45D, 45E, 45F, and 45G are illustrations ofexamples of an end-to-end relay visible coverage areas.

FIGS. 46A and 46B are an illustration of an example of an end-to-endrelay Earth coverage area and North American coverage area,respectively.

FIGS. 47A and 47B are block diagrams of an example forward signal pathand return signal path, respectively, each having selective activationof multiple user-link antenna subsystems.

FIGS. 48A and 48B are an illustration of an example of an end-to-endrelay coverage area that includes multiple, selectively activated usercoverage areas.

FIGS. 49A and 49B are block diagrams of example forward and returnsignal paths, respectively, each having selective activation of multipleuser-link antenna subsystems and multiple feeder-link antenna subsystem.

FIGS. 50A, 50B, and 50C illustrate examples of one or more user coverageareas with multiple access node areas.

FIGS. 51A and 51B show example forward and return signal paths,respectively, each having selective activation of multiple user-linkantenna element arrays and multiple feeder-link antenna element arrays.

FIGS. 52A and 52B show example forward and return receive/transmitsignal paths for concurrent use of multiple AN clusters, respectively.

FIGS. 53A and 53B illustrate example transponders allowing selectivecoupling between multiple feeder-link constituent elements and a singleuser-link constituent element.

FIGS. 54A and 54B illustrate forward and return link transponders,respectively.

FIGS. 55A, 55B, and 55C illustrate example loopback transponders.

FIG. 56A illustrates an end-to-end relay that includes one or morereflectors.

FIG. 56B illustrates an antenna subsystem with multiple feed clusters.

FIG. 57 illustrates an antenna subsystem that includes a compoundreflector.

FIG. 58 shows an end-to-end relay system with portions disposed on oneor more offshore (e.g., fixed or floating) platforms.

FIGS. 59A and 59B are illustrations of examples of end-to-end relayvisible coverage areas supporting distinct frequency ranges.

FIGS. 60A and 60B show example forward/return receive/transmit signalpaths supporting multiple frequency bands.

FIGS. 61A and 61B show example forward/return receive/transmit signalpaths supporting multiple frequency bands.

FIG. 62 shows an example antenna element array with spatiallyinterleaved subsets of constituent antenna elements.

FIGS. 63A and 63B are illustrations of example frequency allocations.

FIGS. 64A and 64B are illustrations of example frequency allocations.

FIGS. 65A and 65B are illustrations of example frequency allocations.

FIGS. 66A and 66B show example forward/return receive/transmit signalpaths.

Reference designators (e.g., 100) are used herein to refer to aspects ofthe drawings. Similar or like aspects are typically shown using likenumbers. A group of similar or like elements may be referred tocollectively by a single reference designator (e.g., 200), whileindividual elements of the group may be referred to by the referencedesignator with an appended letter (e.g., 200 a, 200 b).

The figures are not intended to be exhaustive or to limit the claimedinvention to the precise form disclosed. The disclosed method andapparatus may be practiced with modification and alteration, and thatthe invention is limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION

This detailed description is organized as follows. First, anintroduction to wireless relay communication systems using satellitecommunication and beamforming are described. Second, end-to-endbeamforming is described generally and at the system level usingsatellite end-to-end beamforming as an example, although application ofend-to-end beamforming is not limited to satellite communications.Third, operation of forward and return data is described in context ofend-to-end beamforming. Fourth, end-to-end relays and their antennas aredescribed using a communication satellite as an example. Next, groundnetworks to form the end-to-end beams are described, including relatedaspects, such as delay equalization, feeder-link impairment removal, andbeam weight computation. Finally, end-to-end beamforming with distinctuser-link and feeder-link coverage areas is described, as well assystems with multiple coverage areas.

Satellite Communication

FIG. 1 is an illustration of an example of a hub and spoke satellitecommunication system 100. The satellite serves as an example of awireless relay. Though many examples are described throughout thisdisclosure in context of a satellite or satellite communication system,such examples are not intended to be limited to satellite; any othersuitable wireless relay may be used and operate in a similar fashion.The system 100 comprises a ground-based Earth station 101, acommunication satellite 103, and an Earth transmission source, such as auser terminal 105. A satellite coverage area may be broadly defined asthat area from which, and/or to which, either an Earth transmissionsource, or an Earth receiver, such as a ground-based Earth station or auser terminal, can communicate through the satellite. In some systems,the coverage area for each link (e.g., forward uplink coverage area,forward downlink coverage area, return uplink coverage area, and returndownlink coverage area) can be different. The forward uplink coveragearea and return uplink coverage area are collectively referred to as theuplink satellite coverage area. Similarly, the forward downlink coveragearea and the return downlink coverage area are collectively referred toas the downlink satellite coverage area. While the satellite coveragearea is only active for a satellite that is in service (e.g., in aservice orbit), the satellite can be considered as having (e.g., can bedesigned to have) a satellite antenna pattern that is independent of therelative location of the satellite with respect to the Earth. That is,the satellite antenna pattern is a pattern of distribution of energytransmitted from an antenna of a satellite (either transmitted from orreceived by the antenna of the satellite). The satellite antenna patternilluminates (transmits to or receives from) a particular satellitecoverage area when the satellite is in a service orbit. The satellitecoverage area is defined by the satellite antenna pattern, an orbitalposition and attitude for which the satellite is designed, and a givenantenna gain threshold. In general, the intersection of an antennapattern (at a particular effective antenna gain, e.g. 3 dB, 4 dB, 6 dB10 dB from peak gain) with a particular physical region of interest(e.g., an area on or near the earth surface) defines the coverage areafor the antenna. Antennas can be designed to provide a particularantenna pattern (and/or coverage area) and such antenna patterns can bedetermined computationally (e.g., by analysis or simulation) and/ormeasured experimentally (e.g., on an antenna test range or in actualuse).

While only one user terminal 105 is shown in the figure for the sake ofsimplicity, there are typically many user terminals 105 in the system.The satellite communication system 100 operates as a point tomulti-point system. That is, the Earth station 101 within the satellitecoverage area can send information to, and receive information from, anyof the user terminals 105 within the satellite coverage area. However,the user terminals 105 only communicate with the Earth station 101. TheEarth station 101 receives forward data from a communication network107, modulates the data using a feeder link modem 109 and transmits thedata to the satellite 103 on a forward feeder uplink 111. The satellite103 relays this forward data to user terminals 105 on the forward userdownlink (sometimes called a forward service downlink) 113. In somecases, the forward direction communication from the Earth station 101 isintended for several of the user terminals 105 (e.g., information ismulticast to the user terminals 105). In some cases, the forwardcommunication from the Earth station 101 is intended for only one userterminal 105 (e.g., unicast to a particular user terminal 105). The userterminals 105 transmit return data to the satellite 103 on a return useruplink (sometimes called a return service uplink) 115. The satellite 103relays the return data to the Earth station 101 on a return feederdownlink 117. A feeder-link modem 109 demodulates the return data, whichis forwarded to the communication network 107. This return-linkcapability is generally shared by a number of user terminals 105.

FIG. 2 is a diagram showing an example of one configuration of beamcoverage areas of a satellite to service the continental United States.Seventy beams are shown in the example configuration. A first beam 201covers approximately two thirds of the state of Washington. A secondbeam 203 adjacent to the first beam 201 covers an area immediately tothe east of the first beam 201. A third beam 205 approximately coversOregon to the south of the first beam 201. A fourth beam 207 covers anarea roughly southeast of the first beam 201. Typically, there is someoverlap between adjacent beams. In some cases, a multi-color (e.g., two,three or four-color re-use pattern) is used. In an example of afour-color pattern, the beams 201, 203, 205, 207 are individuallyallocated a unique combination of frequency (e.g., a frequency range orranges or one or more channels) and/or antenna polarization (e.g., insome cases an antenna may be configured to transmit signals with aright-hand circular polarization (RHCP) or a left-hand circularpolarization (LHCP); other polarization techniques are available).Accordingly, there may be relatively little mutual interference betweensignals transmitted on different beams 201, 203, 205, 207. Thesecombinations of frequency and antenna polarization may then be re-usedin the repeating non-overlapping “four-color” re-use pattern. In somesituations, a desired communication capacity may be achieved by using asingle color. In some cases, time sharing among beams and/or otherinterference mitigation techniques can be used.

Within some limits, focusing beams into smaller areas and thusincreasing the number of beams, increases the data capacity of thesatellite by allowing greater opportunity for frequency re-use. However,increasing the number of beams can increase the complexity of thesystem, and in many cases, the complexity of the satellite.

Complexity in the design of a satellite typically results in largersize, more weight, and greater power consumption. Satellites areexpensive to launch into orbit. The cost of launching a satellite isdetermined in part by the weight and size of the satellite. In addition,there are absolute limits on the weight and size of a satellite if thesatellite is to be launched using presently available rocket technology.This leads to tradeoffs between features that may be designed into asatellite. Furthermore, the amount of power that may be provided tocomponents of a satellite is limited. Therefore, weight, size, and powerconsumption are parameters to be considered in the design of asatellite.

Throughout this disclosure, the term receive antenna element refers to aphysical transducer that converts an electro-magnetic signal to anelectrical signal, and the term transmit antenna element refers to aphysical transducer that launches an electro-magnetic signal whenexcited by an electrical signal. The antenna element can include a horn,septum polarized horn (e.g., which may function as two combined elementswith different polarizations), multi-port multi-band horn (e.g.,dual-band 20 GHz/30 GHz with dual polarization LHCP/RHCP), cavity-backedslot, inverted-F, slotted waveguide, Vivaldi, Helical, loop, patch, orany other configuration of antenna element or combination ofinterconnected sub-elements. An antenna element has a correspondingantenna pattern, which describes how the antenna gain varies as afunction of direction (or angle). An antenna element also has a coveragearea which corresponds to an area (e.g., a portion of the Earth surface)or volume (e.g., a portion of the Earth surface plus airspace above thesurface) over which the antenna element provides a desired level of gain(e.g., within 3 dB, 6 dB, 10 dB, or other value relative to a peak gainof the antenna element). The coverage area of the antenna element may bemodified by various structures such as a reflector, frequency selectivesurface, lens, radome, and the like. Some satellites, including thosedescribed herein, can have several transponders, each able toindependently receive and transmit signals. Each transponder is coupledto antenna elements (e.g., a receive element and a transmit element) toform a receive/transmit signal path that has a different radiationpattern (antenna pattern) from the other receive/transmit signal pathsto create unique beams that may be allocated to different beam coverageareas. It is common for a single receive/transmit signal path to beshared across multiple beams using input and/or output multiplexers. Inboth cases, the number of simultaneous beams that may be formed isgenerally limited by the number of receive/transmit signal paths thatare deployed on the satellite.

Beamforming

Beamforming for a communication link may be performed by adjusting thesignal phase (or time delay), and sometimes signal amplitude, of signalstransmitted and/or received by multiple elements of one or more antennaarrays with overlapping coverage areas. In some cases, some or allantenna elements are arranged as an array of constituent receive and/ortransmit elements that cooperate to enable end-to-end beamforming, asdescribed below. For transmissions (from transmit elements of the one ormore antenna arrays), the relative phases, and sometimes amplitudes, ofthe transmitted signals are adjusted, so that the energy transmitted bytransmit antenna elements will constructively superpose at a desiredlocation. This phase/amplitude adjustment is commonly referred to as“applying beam weights” to the transmitted signals. For reception (byreceive elements of the one or more antenna arrays), the relativephases, and sometimes amplitudes, of the received signals are adjusted(i.e., the same or different beam weights are applied) so that theenergy received from a desired location by receive antenna elements willconstructively superpose at those receive antenna elements. In somecases, the beamformer computes the desired antenna element beam weights.The term beamforming may refer in some cases to the application of thebeam weights. Adaptive beamformers include the function of dynamicallycomputing the beam weights. Computing the beam weights may requiredirect or indirect discovery of the communication channelcharacteristics. The processes of beam weight computation and beamweight application may be performed in the same or different systemelements.

The antenna beams may be steered, selectively formed, and/or otherwisereconfigured by applying different beam weights. For example, the numberof active beams, coverage area of beams, size of beams, relative gain ofbeams, and other parameters may be varied over time. Such versatility isdesirable in certain situations. Beamforming antennas can generally formrelatively narrow beams. Narrow beams may allow the signals transmittedon one beam to be distinguished from signals transmitted on the otherbeams (e.g., to avoid interference). Accordingly, narrow beams can allowfrequency and polarization to be re-used to a greater extent than whenlarger beams are formed. For example, beams that are narrowly formed canservice two discontiguous coverage areas that are non-overlapping. Eachbeam can use both a right hand polarization and a left handpolarization. Greater reuse can increase the amount of data transmittedand/or received.

Some satellites use on-board beamforming (OBBF) to electronically steeran array of antenna elements. FIG. 3 is an illustration of a satellitesystem 300 in which the satellite 302 has phased array multi-feed perbeam (MFPB) on-board beamforming capability. In this example, the beamweights are computed at a ground based computation center and thentransmitted to the satellite or pre-stored in the satellite forapplication (not shown). The forward link is shown in FIG. 3 , althoughthis architecture may be used for forward links, return links, or bothforward and return links. Beamforming may be employed on the user link,the feeder link, or both. The illustrated forward link is the signalpath from one of a plurality of gateways (GWs) 304 to one or more of aplurality of user terminals within one or more spot beam coverage areas306. The satellite 302 has a receive antenna array 307, a transmitantenna array 309, a down-converter (D/C) and gain module 311, a receivebeamformer 313, and a transmit beamformer 315. The satellite 302 canform beams on both the feeder link 308 and the user link 310. Each ofthe L elements of the receive array 307 receives K signals from the KGWs 304. For each of the K feeder link beams that are to be created(e.g., one beam per GW 304), a different beam weight is applied (e.g., aphase/amplitude adjustment is made) by the receive beamformer 313 toeach signal received by each of the L receive antenna array elements (ofreceive antenna array 307). Accordingly, for K beams to be formed usinga receive antenna array 307 having L receive antenna elements, Kdifferent beam weight vectors of length L are applied to the L signalsreceived by the L receive antenna array elements. The receive beamformer313 within the satellite 302 adjusts the phase/amplitude of the signalsreceived by the L receive antenna array elements to create K receivebeam signals. Each of the K receive beams are focused to receive asignal from one GW 304. Accordingly, the receive beamformer 313 outputsK receive beam signals to the D/C and gain module 311. One such receivebeam signal is formed for the signal received from each transmitting GW304.

The D/C and gain module 311 down-converts each of the K receive beamsignals and adjusts the gain appropriately. K signals are output fromthe D/C and gain module 311 and coupled to the transmit beamformer 315.The transmit beamformer 315 applies a vector of L weights to each of theK signals for a total of L×K transmit beam weights to form K beams onthe user downlink 310.

In some cases, significant processing capability may be needed withinthe satellite to control the phase and gain of each antenna element thatis used to form the beams. Such processing power increases thecomplexity of the satellite. In some cases, satellites may operate withground-based beamforming (GBBF) to reduce the complexity of thesatellite while still providing the advantage of electronically formingnarrow beams.

FIG. 4 is an illustration of one example of a satellite communicationsystem 400 having forward GBBF. GBBF is performed on the forward userlink 317 via an L element array similar to that described above. Thephases/amplitudes of the signals transmitted on the user link 317 areweighted such that beams are formed. The feeder link 319 uses a SingleFeed per Beam (SFPB) scheme in which each receive and transmit antennaelement of an antenna 324 is dedicated to one feeder link beam.

Prior to transmission from a GW or GWs 304, for each of the K forwardfeeder link beams, a transmit beamformer 321 applies a respective one ofK beam weight vectors, each of length L, to each of K signals to betransmitted. Determining the K vectors of L weights and applying them tothe signals enables K forward beams to be formed on the ground for theforward user downlink 317. On the feeder uplink 319, each of the Ldifferent signals is multiplexed into a frequency division multiplexed(FDM) signal by a multiplexer 323 (or the like). Each FDM signal istransmitted by the GWs 304 to one of the receive antenna elements in theantenna 324 on the feeder link 319. An FDM receiver 325 on the satellite327 receives the signals from the antenna 324. An analog to digitalconverter (A/D) 326 converts the received analog signals to digitalsignals. A digital channel processor 328 demultiplexes the FDM signals,each of which was appropriately weighted by the beamformer 321 fortransmission through one of the L elements of an array of transmitantenna elements of a transmit antenna 329. The digital channelprocessor 328 outputs the signals to a digital to analog converter (D/A)331 to be converted back to analog form. The analog outputs of the D/A331 are up-converted and amplified by an up-converter (U/C) and gainstage 330 and transmitted by the associated element of the transmitantenna 329. A complimentary process occurs in reverse for the returnbeams. Note that in this type of system the FDM feeder link requires Ltimes as much bandwidth as the user beams making it impractical forsystems with wide data bandwidths or systems that have a large number ofelements L.

End-to-End Beamforming Systems

The end-to-end beamforming systems described herein form end-to-endbeams through an end-to-end relay. An end-to-end beamforming system canconnect user terminals with data sources/sinks. In contrast to thebeamforming systems discussed above, in an end-to-end beamformingsystem, beam weights are computed at a central processing system (CPS)and end-to-end beam weights are applied within the ground network(rather than at a satellite). The signals within the end-to-end beamsare transmitted and received at an array of access nodes (ANs), whichmay be satellite access node (SANs). As described above, any suitabletype of end-to-end relays can be used in an end-to-end beamformingsystem, and different types of ANs may be used to communicate withdifferent types of end-to-end relays. The term “central” refers to thefact that the CPS is accessible to the ANs that are involved in signaltransmission and/or reception and does not refer to a particulargeographic location at which the CPS resides. A beamformer within a CPScomputes one set of end-to-end beam weights that accounts for: (1) thewireless signal uplink paths up to the end-to-end relay; (2) thereceive/transmit signal paths through the end-to-end relay; and (3) thewireless signal downlink paths down from the end-to-end relay. The beamweights can be represented mathematically as a matrix. As discussedabove, OBBF and GBBF satellite systems have beam weight vectordimensions set by the number of antenna elements on the satellite. Incontrast, end-to-end beam weight vectors have dimensions set by thenumber of ANs, not the number of elements on the end-to-end relay. Ingeneral, the number of ANs is not the same as the number of antennaelements on the end-to-end relay. Further, the formed end-to-end beamsare not terminated at either transmit or receive antenna elements of theend-to-end relay. Rather, the formed end-to-end beams are effectivelyrelayed, since the end-to-end beams have uplink signal paths, relaysignal paths (via a satellite or other suitable end-to-end relay), anddownlink signal paths.

Because the end-to-end beamforming takes into account both the user linkand the feeder link (as well as the end-to-end relay) only a single setof beam weights is needed to form the desired end-to-end user beams in aparticular direction (e.g., forward user beams or return user beams).Thus, one set of end-to-end forward beam weights (hereafter referred tosimply as forward beam weights) results in the signals transmitted fromthe ANs, through the forward uplink, through the end-to-end relay, andthrough the forward downlink to combine to form the end-to-end forwarduser beams (hereafter referred to as forward user beams). Conversely,signals transmitted from return users through the return uplink, throughthe end-to-end relay, and the return downlink have end-to-end returnbeam weights (hereafter referred to as return beam weights) applied toform the end-to-end return user beams (hereafter referred to as returnuser beams). Under some conditions, it may be very difficult orimpossible to distinguish between the characteristics of the uplink andthe downlink. Accordingly, formed feeder link beams, formed user beamdirectivity, and individual uplink and downlink carrier to interferenceratio (C/I) may no longer have their traditional role in the systemdesign, while concepts of uplink and downlink signal-to-noise ratio(Es/No) and end-to-end C/I may still be relevant.

FIG. 5 is an illustration of an example end-to-end beamforming system500. The system 500 includes: a ground segment 502; an end-to-end relay503; and a plurality of user terminals 517. The ground segment 502comprises M ANs 515, spread geographically over an AN area. The ANs 515cooperate in transmitting forward uplink signals 521 to form user beams519 and return downlink signals 527 are collectively processed torecover return uplink transmissions 525. A set of ANs 515 that arewithin a distinct (e.g., geographically separated or otherwiseorthogonally configured) AN area and cooperate to perform end-to-endbeamforming for forward and/or return user beams is referred to hereinas an “AN cluster.” In some examples, multiple AN clusters in differentAN areas may also cooperate. AN clusters may also be referred to as “ANfarms” or “SAN farms.” ANs 515 and user terminals 517 can becollectively referred to as Earth receivers, Earth transmitters, orEarth transceivers, depending upon the particular functionality atissue, since they are located on, or near, the Earth and both transmitand receive signals. In some cases, user terminals 517 and/or ANs 515can be located in aircraft, watercraft or mounted on landcraft, etc. Insome cases, the user terminals 517 can be geographically distributed.The ANs 515 can be geographically distributed. The ANs 515 exchangesignals with a CPS 505 within the ground segment 502 via a distributionnetwork 518. The CPS 505 is connected to a data source (not shown), suchas, for example, the internet, a video headend or other such entity.

User terminals 517 may be grouped with other nearby user terminals 517(e.g., as illustrated by user terminals 517 a and 517 b). In some cases,such groups of user terminals 517 are serviced by the same user beam andso reside within the same geographic forward and/or return user beamcoverage area 519. A user terminal 517 is within a user beam if the userterminal 517 is within the coverage area serviced by that user beam.While only one such user beam coverage area 519 is shown in FIG. 5 tohave more than one user terminal 517, in some cases, a user beamcoverage area 519 can have any suitable number of user terminals 517.Furthermore, the depiction in FIG. 5 is not intended to indicate therelative size of different user beam coverage areas 519. That is, theuser beam coverage areas 519 may all be approximately the same size.Alternatively, the user beam coverage areas 519 may be of varying sizes,with some user beam coverage areas 519 much larger than others. In somecases, the number of ANs 515 is not equal to the number of user beamcoverage areas 519.

The end-to-end relay 503 relays signals wirelessly between the userterminals 517 and a number of network access nodes, such as the ANs 515shown in FIG. 5 . The end-to-end relay 503 has a plurality of signalpaths. For example, each signal path can include at least one receiveantenna element, at least one transmit antenna element, and at least onetransponder (as is discussed in detail below). In some cases, theplurality of receive antenna elements are arranged to receive signalsreflected by a receive reflector to form a receive antenna array. Insome cases, the plurality of transmit antenna elements is arranged totransmit signals and thus to form a transmit antenna array.

In some cases, the end-to-end relay 503 is provided on a satellite. Inother cases, the end-to-end relay 503 is provided on an aircraft, blimp,tower, underwater structure or any other suitable structure or vehiclein which an end-to-end relay 503 can reside. In some cases, the systemuses different frequency ranges (in the same or different frequencybands) for the uplinks and downlinks. In some cases, the feeder linksand user links are in different frequency ranges. In some cases, theend-to-end relay 503 acts as a passive or active reflector.

As described herein, various features of the end-to-end relay 503 enableend-to-end beamforming. One feature is that the end-to-end relay 503includes multiple transponders that, in the context of end-to-endbeamforming systems, induce multipath between the ANs 515 and the userterminals 517. Another feature is that the antennas (e.g., one or moreantenna subsystems) of the end-to-end relay 503 contribute to end-to-endbeamforming, so that forward and/or return user beams are formed whenproperly beam-weighted signals are communicated through the multipathinduced by the end-to-end relay 503. For example, during forwardcommunications, each of multiple transponders receives a respectivesuperposed composite of (beam weighted) forward uplink signals 521 frommultiple (e.g., all) of the ANs 515 (referred to herein as compositeinput forward signals), and the transponders output correspondingcomposite signals (referred to herein as forward downlink signals). Eachof the forward downlink signals can be a unique composite of thebeam-weighted forward uplink signals 521, which, when transmitted by thetransmit antenna elements of the end-to-end relay 503, superpose to formthe user beams 519 in desired locations (e.g., recovery locations withinforward user beams, in this case). Return end-to-end beamforming issimilarly enabled. Thus, the end-to-end relay 503 can cause multiplesuperpositions to occur, thereby enabling end-to-end beamforming overinduced multipath channels.

Return Data

FIG. 6 is an illustration of an example model of signal paths forsignals carrying return data on the end-to-end return link. Return datais the data that flows from user terminals 517 to the ANs 515. Signalsin FIG. 6 flow from right to left. The signals originate with userterminals 517. The user terminals 517 transmit return uplink signals 525(which have return user data streams) up to the end-to-end relay 503.Return uplink signals 525 from user terminals 517 in K user beamcoverage areas 519 are received by an array of L receive/transmit signalpaths 1702. In some cases, an uplink coverage area for the end-to-endrelay 503 is defined by that set of points from which all of the Lreceive antenna elements 406 can receive signals. In other cases, therelay coverage area is defined by that set of points from which a subset(e.g., a desired number more than 1, but less than all) of the L receiveantenna elements 406 can receive signals. Similarly, in some cases, thedownlink coverage area is defined by the set of points to which all ofthe L transmit antenna elements 409 can reliably send signals. In othercases, the downlink coverage area for the end-to-end relay 503 isdefined as that set of points to which a subset of the transmit antennaelements 409 can reliably send signals. In some cases, the size of thesubset of either receive antenna elements 406 or transmit antennaelements 409 is at least four. In other cases, the size of the subset is6, 10, 20, 100, or any other number that provides the desired systemperformance.

For the sake of simplicity, some examples are described and/orillustrated as all L receive antenna elements 406 receiving signals fromall points in the uplink coverage area and/or all L transmit antennaelements 409 transmitting to all points in the downlink coverage area.Such descriptions are not intended to require that all L elementsreceive and/or transmit signals at a significant signal level. Forexample, in some cases, a subset of the L receive antenna elements 406receives an uplink signal (e.g., a return uplink signal 525 from a userterminal 517, or a forward uplink signal 521 from an AN 515), such thatthe subset of receive antenna elements 406 receives the uplink signal ata signal level that is close to a peak received signal level of theuplink signal (e.g., not substantially less than the signal levelcorresponding to the uplink signal having the highest signal level);others of the L receive antenna elements 406 that are not in the subsetreceive the uplink signal at an appreciably lower level (e.g., far belowthe peak received signal level of the uplink signal). In some cases, theuplink signal received by each receive antenna element of a subset is ata signal level within 10 dB of a maximum signal level received by any ofthe receive antenna elements 406. In some cases, the subset includes atleast 10% of the receive antenna elements 406. In some cases, the subsetincludes at least 10 receive antenna elements 406.

Similarly, on the transmit side, a subset of the L transmit antennaelements 409 transmits a downlink signal to an Earth receiver (e.g., areturn downlink signal 527 to an AN 515, or a forward downlink signal522 to a user terminal 517), such that the subset of transmit antennaelements 409 transmits the downlink signal to the receiver with areceived signal level that is close to a peak transmitted signal levelof the downlink signal (e.g., not substantially less than the signallevel corresponding to the downlink signal having the highest receivedsignal level); others of the L transmit antenna elements 409 that arenot in the subset transmit the downlink signal such that it is receivedat an appreciably lower level (e.g., far below the peak transmittedsignal level of the downlink signal). In some cases, the signal level iswithin 3 dB of a signal level corresponding to a peak gain of thetransmit antenna element 409. In other cases, the signal level is within6 dB of the signal level corresponding to a peak gain of the transmitantenna element 409. In yet other cases, the signal level is within 10dB of the signal level corresponding to a peak gain of the transmitantenna element 409.

In some cases, the signal received by each receive antenna element 406originates at the same source (e.g., one of the user terminals 517) dueto overlap in the receive antenna pattern of each receive antennaelement. However, in some cases, there may be points within theend-to-end relay coverage area at which a user terminal is located andfrom which not all of the receive antenna elements can receive thesignal. In some such cases, there may be a significant number of receiveantenna elements that do not (or cannot) receive the signal from userterminals that are within the end-to-end relay coverage area. However,as described herein, inducing multipath by the end-to-end relay 503 canrely on receiving the signal by at least two receive elements.

As shown in FIG. 6 and discussed in greater detail below, in some cases,a receive/transmit signal path 1702 comprises a receive antenna element406, a transponder 410, and a transmit antenna element 409. In suchcases, the return uplink signals 525 are received by each of a pluralityof transponders 410 via a respective receive antenna element 406. Theoutput of each receive/transmit signal path 1702 is a return downlinksignal 527 corresponding to a respective composite of received returnuplink signals. The return downlink signal is created by thereceive/transmit signal path 1702. The return downlink signal 527 istransmitted to the array of M ANs 515. In some cases, the ANs 515 areplaced at geographically distributed locations (e.g., reception orrecovery locations) throughout the end-to-end relay coverage area. Insome cases, each transponder 410 couples a respective one of the receiveantenna elements 406 with a respective one of the transmit antennaelements 409. Accordingly, there are L different ways for a signal toget from a user terminal 517 located in a user beam coverage area 519 toa particular AN 515. This creates L paths between a user terminal 517and an AN 515. The L paths between one user terminal 517 and one AN 515are referred to collectively as an end-to-end return multipath channel1908 (see FIG. 8 ). Accordingly receiving the return uplink signal 525from a transmission location within a user beam coverage area 519,through the L transponders 410, creates L return downlink signals 527,each transmitted from one of the transponders 410 (i.e., through Lcollocated communication paths). Each end-to-end return multipathchannel 1908 is associated with a vector in the uplink radiation matrixA_(r), the payload matrix E, and a vector in downlink radiation matrixC_(t). Note that due to antenna element coverage patterns, in somecases, some of the L paths may have relatively little energy (e.g., 6dB, 10 dB, 20 dB, 30 dB, or any other suitable power ratio less thanother paths). A superposition 1706 of return downlink 527 signal isreceived at each of the ANs 515 (e.g., at M geographically distributedreception or recovery locations). Each return downlink signal 527comprises a superposition of a plurality of the transmitted returndownlink signals 527, resulting in a respective composite return signal.The respective composite return signals are coupled to the returnbeamformer 531 (see FIGS. 5 and 29 ).

FIG. 7 illustrates an example end-to-end return link 523 from one userterminal 517 located within a user beam coverage area 519 to the ANs515. The return uplink signal 525 transmitted from the user terminal 517is received by the array of L receive antenna elements 406 on theend-to-end relay 503 (e.g., or received by a subset of the L receiveantenna elements 406).

Ar is the L×K return uplink radiation matrix. The values of the returnuplink radiation matrix model the signal path from a reference locationin the user beam coverage area 519 to the end-to-end relay receiveantenna elements 406. For example, Ar_(L,1) is the value of one elementof the return uplink radiation matrix (i.e. the amplitude and phase ofthe path) from a reference location in the 1^(st) user beam coveragearea 519 to the L^(th) receive antenna element. In some cases, all ofthe values in the return uplink radiation matrix Ar may be non-zero(e.g., there is a significant signal path from the reference location toeach of the receive antenna elements of the receive antenna array).

E (dimension L×L) is the payload matrix and provides the model(amplitude and phase) of the paths from the receive antenna elements 406to the transmit antenna elements 409. A “payload” of an end-to-end relay503, as used herein, generally includes the set of components of theend-to-end relay 503 that affect, and/or are affected by, signalcommunications as they are received by, relayed through, and transmittedfrom the end-to-end relay 503. For example, an end-to-end relay payloadcan include antenna elements, reflectors, transponders, etc.; but theend-to-end relay can further include batteries, solar cells, sensors,and/or other components not considered herein as part of the payload(since they do not affect signals when operating normally).Consideration of the set of components as a payload can enablemathematically modeling the overall impact of the end-to-end relay as asingle payload matrix E). The predominant path from each receive antennaelement 406 to each corresponding transmit antenna element 409 ismodeled by the value that lies on the diagonal of the payload matrix E.Assuming there is no crosstalk between receive/transmit signal paths,the off-diagonal values of the payload matrix are zero. In some cases,the crosstalk may not be zero. Isolating the signal paths from eachother will minimize crosstalk. In some cases, since the crosstalk isnegligible, the payload matrix E can be estimated by a diagonal matrix.In some cases, the off-diagonal values (or any other suitable values) ofthe payload matrix can be treated as zero, even where there is somesignal impact corresponding to those values, to reduce mathematicalcomplexity and/or for other reasons.

Ct is the M×L return downlink radiation matrix. The values of the returndownlink radiation matrix model the signal paths from the transmitantenna elements 409 to the ANs 515. For example, Ct_(3,2) is the valueof the return downlink radiation matrix (e.g., the gain and phase of thepath) from the second transmit antenna element 409 _(b) to the third AN515 _(c). In some cases, all of the values of the downlink radiationmatrix Ct may be non-zero. In some cases, some of the values of thedownlink radiation matrix Ct are essentially zero (e.g., the antennapattern established by a corresponding transmit antenna elements 409 ofthe transmit antenna array is such that the transmit antenna element 409does not transmit useful signals to some of the ANs 515).

As can be seen in FIG. 7 , the end-to-end return multipath channel froma user terminal 517 in a particular user beam coverage area 519 to aparticular AN 515 is the sum of the L different paths. The end-to-endreturn multipath channel has multipath induced by the L unique pathsthrough the transponders 410 in the end-to-end relay. As with manymultipath channels, the paths' amplitudes and phases can add upfavorably (constructively) to produce a large end-to-end channel gain orunfavorably (destructively) to produce a low end-to-end channel gain.When the number of different paths, L, between a user terminal and an ANis large, the end-to-end channel gain can have a Rayleigh distributionof the amplitude. With such a distribution, it is not uncommon to seesome end-to-end channel gains from a particular user terminal 517 to aparticular AN 515 that are 20 dB or more below the average level of thechannel gain from a user terminal 517 to an AN 515. This end-to-endbeamforming system intentionally induces a multipath environment for theend-to-end path from any user terminal to any AN.

FIG. 8 is a simplified illustration of an example model of all theend-to-end return multipath channels from user beam coverage areas 519to ANs 515. There are M×K such end-to-end return multipath channels inthe end-to-end return link (i.e., M from each of the K user beamcoverage areas 519). Channels 1908 connect user terminals in one userbeam coverage area 519 to one AN 515 over L different receive/transmitsignal paths 1702, each path going through a different one of the Lreceive/transmit signal paths (and associated transponders) of therelay. While this effect is referred to as “multipath” herein, thismultipath differs from conventional multipath (e.g., in a mobile radioor multiple-input multiple-output (MIMO) system), as the multiple pathsherein are intentionally induced (and, as described herein, affected) bythe L receive/transmit signal paths. Each of the M×K end-to-end returnmultipath channels that originate from a user terminal 517 within aparticular user beam coverage area 519 can be modeled by an end-to-endreturn multipath channel. Each such end-to-end return multipath channelis from a reference (or recovery) location within the user beam coveragearea 519 to one of the ANs 515.

Each of the M×K end-to-end return multipath channels 1908 may beindividually modeled to compute a corresponding element of an M×K returnchannel matrix Hret. The return channel matrix Hret has K vectors, eachhaving dimensionality equal to M, such that each vector models theend-to-end return channel gains for multipath communications between areference location in one of a respective K user beam coverage areas andthe M ANs 515. Each end-to-end return multipath channel couples one ofthe M ANs 515 with a reference location within one of K return userbeams via L transponders 410 (see FIG. 7 ). In some cases, only a subsetof the L transponders 410 on the end-to-end relay 503 is used to createthe end-to-end return multipath channel (e.g., only a subset isconsidered to be in the signal path by contributing significant energyto the end-to-end return multipath channel). In some cases, the numberof user beams K is greater than the number of transponders L that is inthe signal path of the end-to-end return multipath channel. Furthermore,in some cases, the number of ANs M is greater than the number oftransponders L that is in the signal path of the end-to-end returnmultipath channel 1908. In an example, the element Hret_(4,2) of thereturn channel matrix Hret is associated with the channel from areference location in the second user beam coverage area 1903 to thefourth AN 1901. The matrix Hret models the end-to-end channel as theproduct of the matrices Ct×E×Ar (see FIG. 6 ). Each element in Hretmodels the end-to-end gain of one end-to-end return multipath channel1908. Due to the multipath nature of the channel, the channel can besubject to a deep fade. Return user beams may be formed by the CPS 505.The CPS 505 computes return beam weights based on the model of these M×Ksignal paths and forms the return user beams by applying the return beamweights to the plurality of composite return signals, each weight beingcomputed for each end-to-end return multipath channel that couples theuser terminals 517 in one user beam coverage area with one of theplurality of ANs 515. In some cases, the return beam weights arecomputed before receiving the composite return signal. There is oneend-to-end return link from each of the K user beam coverage areas 519to the M ANs 515. The weighting (i.e., the complex relativephase/amplitude) of each of the signals received by the M ANs 515 allowsthose signals to be combined to form a return user beam using thebeamforming capability of the CPS 505 within the ground segment 502. Thecomputation of the beam weight matrix is used to determine how to weighteach end-to-end return multipath channel 1908, to form the plurality ofreturn user beams, as described in more detail below. User beams are notformed by directly adjusting the relative phase and amplitude of thesignals transmitted by one end-to-end relay antenna element with respectto the phase and amplitude of the signals transmitted by the otherend-to-end relay antenna elements. Rather, user beams are formed byapplying the weights associated with the M×K channel matrix to the M ANsignals. It is the plurality of ANs that provide the receive pathdiversity, single transmitter (user terminal) to multiple receivers(ANs), to enable the successful transmission of information from anyuser terminal in the presence of the intentionally induced multipathchannel.

Forward Data

FIG. 9 is an illustration of an example model of signal paths forsignals carrying forward data on the end-to-end forward link 501.Forward data is the data that flows from ANs 515 to user terminals 517.Signals in this figure flow from right to left. The signals originatewith M ANs 515, which are located in the footprint of the end-to-endrelay 503. There are K user beam coverage areas 519. Signals from eachAN 515 are relayed by L receive/transmit signal paths 2001.

The receive/transmit signal paths 2001 transmit a relayed signal to userterminals 517 in user beam coverage areas 519. Accordingly, there may beL different ways for a signal to get from a particular AN 515 to a userterminal 517 located in a user beam coverage area 519. This creates Lpaths between each AN 515 and each user terminal 517. Note that due toantenna element coverage patterns, some of the L paths may have lessenergy than other paths.

FIG. 10 illustrates an example end-to-end forward link 501 that couplesa plurality of access nodes at geographically distributed locations witha user terminal 517 in a user beam (e.g., located at a recovery locationwithin a user beam coverage area 519) via an end-to-end relay 503. Insome cases, the forward data signal is received at a beamformer prior togenerating forward uplink signals. A plurality of forward uplink signalsis generated at the beamformer and communicated to the plurality of ANs515. For example, each AN 515 receives a unique (beam weighted) forwarduplink signal generated according to beam weights corresponding to thatAN 515. Each AN 515 has an output that transmits a forward uplink signalvia one of M uplinks. Each forward uplink signal comprises a forwarddata signal associated with the forward user beam. The forward datasignal is “associated with” the forward user beam, since it is intendedto be received by user terminals 517 serviced by the user beam. In somecases, the forward data signal comprises two or more user data streams.The user data streams can be multiplexed together by time-division orfrequency-division multiplexing, etc. In some cases, each user datastream is for transmission to one or more of a plurality of userterminals within the same forward user beam.

As is discussed in greater detail below, each forward uplink signal istransmitted in a time-synchronized manner by its respective transmittingAN 515. The forward uplink signals 521 transmitted from the ANs 515 arereceived by a plurality of transponders 410 on the end-to-end relay 503via receive antenna elements 406 on the end-to-end relay 503. Thesuperposition 550 of the forward uplink signals 521 received fromgeographically distributed locations creates a composite input forwardsignal 545. Each transponder 410 concurrently receives a composite inputforward signal 545. However, each transponder 410 will receive thesignals with slightly different timing due to the differences in thelocation of the receive antenna element 406 associated with eachtransponder 401.

Cr is the L×M forward uplink radiation matrix. The values of the forwarduplink radiation matrix model the signal path (amplitude and phase) fromthe ANs 515 to the receive antenna elements 406. E is the L×L payloadmatrix and provides the model of the transponder signal paths from thereceive antenna elements 406 to the transmit antenna elements 409. Thedirect path gain from each receive antenna element 406 through acorresponding one of a plurality of transponders to each correspondingtransmit antenna element 409 is modeled by the diagonal values of thepayload matrix. As noted above with respect to the return link, assumingthere is no cross-talk between antenna elements, the off-diagonalelements of the payload matrix are zero. In some cases, the crosstalkmay not be zero. Isolating the signal paths from each other willminimize crosstalk. In this example, each of the transponders 410couples a respective one of the receive antenna elements 406 with arespective one of the transmit antenna elements 409. Accordingly, aforward downlink signal 522 output from each of the transponders 410 istransmitted by each of the plurality of transponders 410 (see FIG. 9 )via the transmit antenna elements 409, such that the forward downlinksignals 522 form a forward user beam (by constructively anddestructively superposing in desired geographic recovery locations toform the beam). In some cases, a plurality of user beams is formed, eachcorresponding to a geographic user beam coverage area 519 that servicesa respective set of user terminals 517 within the user beam coveragearea 519. The path from the first transmit antenna element 409 a (seeFIG. 10 ) to a reference (or recovery) location in the first user beamcoverage area 519 is given in the At₁₁ value of the forward downlinkradiation matrix. As noted with regard to the return link, thisend-to-end beamforming system intentionally induces a multipathenvironment for the end-to-end path from any AN 515 to any user terminal517. In some cases, a subset of the transmit antenna elements 409transmits forward downlink signals 522 with significant energy to a userterminal 517. The user terminal 517 (or, more generally, a reference orrecovery location in the user beam coverage area 519 for receivingand/or recovery) receives the plurality of forward downlink signals 522and recovers at least a portion of the forward data signal from thereceived plurality of forward downlink signals 522. The transmittedforward downlink signals 522 may be received by the user terminal 517 ata signal level that is within 10 dB of a maximum signal level from anyof the other signals transmitted by the transmit antenna elements 409within the subset. In some cases, the subset of transmit antennaelements includes at least 10% of the plurality of transmit antennaelements present in the end-to-end relay 503. In some cases, the subsetof transmit antenna elements include at least 10 transmit antennaelements, regardless of how many transmit antenna elements 409 arepresent in the end-to-end relay 503. In one case, receiving theplurality of forward downlink signals comprises receiving asuperposition 551 of the plurality of forward downlink signals.

FIG. 11 is a simplified illustration of a model of all the end-to-endforward multipath channels 2208 from the M ANs 515 to the K user beamcoverage areas 519. As shown in FIG. 11 , there is an end-to-end forwardmultipath channel 2208 that couples each AN 515 to each user beamcoverage area 519. Each channel 2208 from one AN 515 to one user beamcoverage area 519 has multipath induced as a result of L unique pathsfrom the AN 515 through the plurality of transponders to the user beamcoverage area 519. As such, the K×M multipath channels 2208 may beindividually modeled and the model of each serves as an element of a K×Mforward channel matrix Hfwd. The forward channel matrix Hfwd has Mvectors, each having dimensionality equal to K, such that each vectormodels the end-to-end forward gains for multipath communications betweena respective one of the M ANs 515 and reference (or recovery) locationsin K forward user beam coverage areas. Each end-to-end forward multipathchannel couples one of the M ANs 515 with user terminals 517 serviced byone of K forward user beams via L transponders 410 (see FIG. 10 ). Insome cases, only a subset of the L transponders 410 on the end-to-endrelay 503 are used to create the end-to-end forward multipath channel(i.e., are in the signal path of the end-to-end forward multipathchannel). In some cases, the number of user beams K is greater than thenumber of transponders L that are in the signal path of the end-to-endforward multipath channel. Furthermore, in some cases, the number of ANsM is greater than the number of transponders L that are in the signalpath of the end-to-end forward multipath channel.

Hfwd may represent the end-to-end forward link as the product ofmatrices At×E×Cr. Each element in Hfwd is the end-to-end forward gaindue to the multipath nature of the path and can be subject to a deepfade. An appropriate beam weight may be computed for each of theplurality of end-to-end forward multipath channels 2208 by the CPS 505within the ground segment 502 to form forward user beams from the set ofM ANs 515 to each user beam coverage area 519. The plurality of ANs 515provide transmit path diversity, by using multiple transmitters (ANs) toa single receiver (user terminal), to enable the successful transmissionof information to any user terminal 517 in the presence of theintentionally induced multipath channel.

Combined Forward and Return Data

FIG. 12 illustrates an example end-to-end relay supporting both forwardand return communications. In some cases, the same end-to-end relaysignal paths (e.g., set of receive antenna elements, transponders, andtransmit antenna elements) may be used for both the end-to-end forwardlink 501 and the end-to-end return link 523. Some other cases includeforward link transponders and return link transponders, which may or maynot share receive and transmit antenna elements. In some cases, thesystem 1200 has a plurality of ANs and user terminals that are locatedin the same general geographic region 1208 (which may be, for example, aparticular state, an entire country, a region, an entire visible area,or any other suitable geographic region 1208). A single end-to-end relay1202 (disposed on a satellite or any other suitable end-to-end relay)receives forward uplink signals 521 from ANs and transmits forwarddownlink signals 522 to user terminals. At alternate times, or onalternate frequencies, the end-to-end relay 1202 also receives returnuplink signals 525 from the user terminals and transmits return downlinksignals 527 to the ANs. In some cases, the end-to-end relay 1202 isshared between forward and return data using techniques such as timedomain duplexing, frequency domain duplexing, and the like. In somecases, time domain duplexing between forward and return data uses thesame frequency range: forward data is transmitted during different(non-overlapping) time intervals than those used for transmitting returndata. In some cases, with frequency domain duplexing, differentfrequencies are used for forward data and return data, therebypermitting concurrent, non-interfering transmission of forward andreturn data.

FIG. 13 is an illustration of an uplink frequency range being dividedinto two portions. The lower-frequency (left) portion of the range isallocated to the forward uplink and the upper-frequency (right) portionof the range is allocated to the return uplink. The uplink range may bedivided into multiple portions of either forward or return data.

FIG. 14 is an illustration of the forward data and return data beingtime division multiplexed. A data frame period is shown in which forwarddata is transported during the first time interval of the frame, whilereturn data is transported during the last time interval of the frame.The end-to-end relay receives from one or more access nodes during afirst (forward) receive time interval and from one or more userterminals during a second (return) receive time interval that doesn'toverlap the first receive time interval. The end-to-end relay transmitsto one or more user terminals during a first (forward) transmit timeinterval and to one or more access nodes during a second (return)transmit time interval that doesn't overlap the first receive timeinterval. The data frame may be repeated or may change dynamically. Theframe may be divided into multiple (e.g., non-contiguous) portions forforward and return data.

End-to-End Beamforming Satellites

In some cases, the end-to-end relay 503 is implemented on a satellite,so that the satellite is used to relay the signals from the ANs (whichcan be referred to as satellite access nodes (SANs) in such cases) tothe user terminals and vice versa. In some cases, the satellite is ingeostationary orbit. An example satellite operating as an end-to-endrelay has an array of receive antenna elements, an array of transmitantenna elements, and a number of transponders that connect the receiveantenna elements to the transmit antenna elements. The arrays have alarge number of antenna elements with overlapping antenna elementcoverage areas, similar to traditional single link phased arrayantennas. It is the overlapping antenna element coverage areas on boththe transmit antenna elements and receive antenna elements that createthe multipath environment previously described. In some cases, theantenna patterns established by the corresponding antenna elements, andthose that result in the overlapping antenna element coverage areas(e.g., overlapping component beam antenna patterns), are identical. Forthe purposes of this disclosure, the term “identical” means that theyfollow essentially the same distribution of power over a given set ofpoints in space, taking the antenna element as the point of referencefor locating the points in space. It is very difficult to be perfectlyidentical. Therefore, patterns that have relatively small deviationsfrom one pattern to another are within the scope of “identical”patterns. In other cases, receive component beam antenna patterns maynot be identical, and in fact may be significantly different. Suchantenna patterns may yet result in overlapping antenna element coverageareas, however, those resulting coverage areas will not be identical.

Antenna types include, but are not limited to, array fed reflectors,confocal arrays, direct radiating arrays and other forms of antennaarrays. Each antenna can be a system including additional opticalcomponents to aid in the receipt and/or transmission of signals, such asone or more reflectors. In some cases, a satellite includes componentsthat assist in system timing alignment and beamforming calibration.

FIG. 15 is a diagram of an example satellite 1502 that can be used as anend-to-end relay 503. In some cases, the satellite 1502 has an array fedreflector transmit antenna 401 and an array fed reflector receiveantenna 402. The receive antenna 402 comprises a receive reflector (notshown) and an array of receive antenna elements 406. The receive antennaelements 406 are illuminated by the receive reflector. The transmitantenna 401 comprises a transmit reflector (not shown) and an array oftransmit antenna elements 409. The transmit antenna elements 409 arearranged to illuminate the transmit reflector. In some cases, the samereflector is used for both receive and transmit. In some cases, one portof the antenna element is used for receiving and another port fortransmission. Some antennas have the ability to distinguish betweensignals of different polarizations. For example, an antenna element caninclude four waveguide ports for right-hand circular polarization (RHCP)receive, left-hand circular polarization (LHCP) receive, RHCP transmit,and LHCP transmit, respectively. In some cases, dual polarizations maybe used to increase capacity of the system; in other cases, singlepolarization may be used to reduce interference (e.g., with othersystems using a different polarization).

The example satellite 1502 also comprises a plurality of transponders410. A transponder 410 connects the output from one receive antennaelement 406 to the input of a transmit antenna element 409. In somecases, the transponder 410 amplifies the received signal. Each receiveantenna element outputs a unique received signal. In some cases, asubset of receive antenna elements 406 receive a signal from an Earthtransmitter, such as either a user terminal 517 in the case of a returnlink signal or an AN 515 in the case of a forward link signal. In someof these cases, the gain of each receive antenna element in the subsetfor the received signal is within a relatively small range. In somecases, the range is 3 dB. In other cases, the range is 6 dB. In yetother cases, the range is 10 dB. Accordingly, the satellite will receivea signal at each of a plurality of receive antenna elements 406 of thesatellite, the communication signal originating from an Earthtransmitter, such that a subset of the receive antenna elements 406receives the communication signal at a signal level that is notsubstantially less than a signal level corresponding to a peak gain ofthe receive antenna element 406.

In some cases, at least 10 transponders 410 are provided within thesatellite 1502. In another case, at least 100 transponders 410 areprovided in the satellite 1502. In yet another case, the number oftransponders per polarity may be in the range of 2, 4, 8, 16, 32, 64,128, 256, 512, 1024 or numbers in-between or greater. In some cases, thetransponder 410 includes a low noise amplifier (LNA) 412, a frequencyconverter and associated filters 414 and a power amplifier (PA) 420. Insome cases in which the uplink frequency and downlink frequency are thesame, the transponder does not include a frequency converter. In othercases, the plurality of receive antenna elements operate at a firstfrequency. Each receive antenna element 406 is associated with onetransponder 410. The receive antenna element 406 is coupled to the inputof the LNA 412. Accordingly, the LNA independently amplifies the uniquereceived signal provided by the receive antenna element associated withthe transponder 410. In some cases, the output of the LNA 412 is coupledto the frequency converter 414. The frequency converter 414 converts theamplified signal to a second frequency.

The output of the transponder is coupled to an associated one of thetransmit antenna elements. In these examples, there is a one to onerelationship between a transponder 410, an associated receive antennaelement 406, and an associated transmit antenna element 409, such thatthe output of each receive antenna element 406 is connected to the inputof one and only one transponder and the output of that transponder isconnected to the input of one and only one transmit antenna element.

FIG. 16 is an illustration of an example transponder 410. Thetransponder 410 can be an example of a transponder of an end-to-endrelay 503, as described above (e.g., the satellite 1502 of FIG. 15 ). Inthis example, the transponder includes a phase shifter 418 in additionto the LNA 412, frequency converter and associated filters 414, andpower amplifier (PA) of transponder 410. As illustrated in FIG. 16 , theexample transponder 410 can also be coupled with a phase shiftcontroller 427. For example, the phase shift controller 427 can becoupled (directly or indirectly) with each of some or all of thetransponders of an end-to-end relay 503, so that the phase shiftcontroller 427 can individually set the phases for each transponder. Thephase shifters may be helpful for calibration, for example, as discussedbelow.

Antennas

To create the multipath environment, antenna element coverage areas canoverlap with antenna element coverage areas of at least one otherantenna element of the same polarity, frequency, and type (transmit orreceive, respectively). In some cases, a plurality of receive componentbeam antenna patterns, operable at the same receive polarization andreceive frequency (e.g., having at least a portion of the receivefrequency in common), overlap with one another. For example, in somecases, at least 25% of the receive component beam antenna patterns,operable at the same receive polarization and receive frequency (e.g.,having at least a portion of the receive frequency in common), overlapwith at least five other receive component beam antenna patterns of thereceive antenna elements. Similarly, in some cases, at least 25% of thetransmit component beam antenna patterns, operable at the same transmitpolarization and transmit frequency (e.g., having at least a portion ofthe transmit frequency in common), overlap with at least five othertransmit component beam antenna patterns. The amount of overlap willvary from system to system. In some cases, at least one of the receiveantenna elements 406 has component beam antenna patterns that overlapwith the antenna patterns of other receive antenna elements 406 operableat the same receive frequency (e.g., having at least a portion of thereceive frequency in common) and same receive polarization. Therefore,at least some of the plurality of receive antenna elements are capableof receiving the same signals from the same source. Similarly, at leastone of the transmit antenna elements 409 has a component beam antennapattern that overlaps with the antenna patterns of other transmitantenna elements 409 operable at the same transmit frequency (e.g.,having at least a portion of the transmit frequency in common) andtransmit polarization. Therefore, at least some of the plurality oftransmit antenna elements are capable of transmitting signals having thesame frequency at the same polarization to the same receiver. In somecases, overlapping component beam antenna patterns may have gains thatdiffer by less than 3 dB (or any other suitable value) over a commongeographic area. The antenna elements, whether receive or transmit, mayhave a broad component beam antenna pattern, and thus a relatively broadantenna element coverage area. In some cases, signals transmitted by anEarth transmitter, such as a user terminal 517 or access node 515, arereceived by all of the receive antenna elements 406 of the end-to-endrelay (e.g., satellite). In some cases, a subset of the elements 406receives the signals from an Earth transmitter. In some cases, thesubset includes at least 50% of the receive antenna elements. In othercases, the subset includes at least 75% of the receive antenna elements.In still other cases, the subset includes at least 90% (e.g., up to andincluding all) of the receive antenna elements. Different subsets of thereceive antenna elements 406 may receive signals from different Earthtransmitters. Similarly, in some cases, a subset of the elements 409transmits signals that may be received by a user terminal 517. In somecases, the subset includes at least 50% of the transmit antennaelements. In other cases, the subset includes at least 75% of thetransmit antenna elements. In still other cases, the subset includes atleast 90% (e.g., up to and including all) of the transmit antennaelements. Different subsets of the elements 409 may transmit signalsthat are received by different user terminals. Furthermore, userterminals may be within several formed user beam coverage areas 519. Forthe purpose of this disclosure, an antenna pattern is a pattern ofdistribution of energy transmitted to, or received from, an antenna. Insome cases, the energy may be directly radiated from/to the antennaelement. In other cases, the energy from one or more transmit antennaelements may be reflected by one or more reflectors that shape theantenna element pattern. Similarly, a receive element may receive energydirectly, or after the energy has reflected off one or more reflectors.In some cases, antennas can be made up of several elements, each havinga component beam antenna pattern that establishes a correspondingantenna element coverage area. Similarly, all or a subset of receive andtransmit antenna elements that receive and transmit signals to ANs 515may overlap, such that a plurality of receive antenna elements receivessignals from the same AN 515 and/or a plurality of transmit antennaelements transmits signals to the same AN 515.

FIG. 17 is an illustration of component beam antenna patterns producedby several antenna elements (either receive antenna elements 406 ortransmit antenna elements 409) that intersect at the 3 dB points. Thecomponent beam antenna pattern 1301 of a first antenna element has peakcomponent beam antenna gain along the boresight 1303. The component beamantenna pattern 1301 is shown to attenuate about 3 dB before itintersects with the component beam antenna pattern 1305. Since each pairof two adjacent component beam antenna patterns overlap about the 3 dBline 1307 for only a relatively small portion of the component beamantenna pattern, the antenna elements that produce these component beamantenna patterns are considered not to be overlapping.

FIG. 18 shows idealized 3 dB antenna contours 3901, 3902, 3903 ofseveral elements 406, 409 with the peak gain designated with the letter‘x’. The contours 3901, 3902, 3903 are referred to herein as “idealized”because the contours are shown as circular for the sake of simplicity.However, the contours 3901, 3902, 3903 need not be circular. Eachcontour indicates the place at which the transmitted or received signalis 3 dB below the peak level. Outside the contour, the signal is morethan 3 dB below the peak. Inside the contour, the signal is less than 3dB below the peak (i.e., within 3 dB of the peak). In a system in whichthe coverage area of a receive component beam antenna pattern is allpoints for which the receive component beam antenna gain is within 3 dBof peak receive component beam antenna gain, the area inside the contouris referred to as the antenna element coverage area. The 3 dB antennacontour for each element 406, 409 is not overlapping. That is, only arelatively small portion of the area inside the 3 dB antenna contour3901 overlaps with the area that is inside the adjacent 3 dB antennapatterns 3902, 3903.

FIG. 19 is an illustration of the antenna patterns 1411, 1413, 1415 ofseveral antenna elements (either receive antenna elements 406 ortransmit antenna elements 409). In contrast to the component beamantenna patterns of FIG. 17 , the component beam antenna patterns shownin FIG. 19 intersect 1417 above the 3 dB line 1307.

FIG. 20A through FIG. 20E illustrate 3 dB antenna contours for severalantenna elements 406, 409 with the beam center point (peak gain)designated with the letter ‘x’. FIG. 20A shows the particular antennacontour 1411 of a first antenna element 406. FIG. 20B shows the 3 dBantenna contours 1411, 1413 for two particular elements 406. FIG. 20Cshows the 3 dB antenna contours for three elements 406. FIG. 20D showsthe 3 dB antenna contours for four antenna elements 406. FIG. 20E showsthe 3 dB antenna contours for an array of 16 antenna elements 406. The 3dB antenna contours are shown to overlap 1418 (e.g., 16 such 3 dBantenna contours are shown). The antenna elements in either the receiveor transmit antenna may be arranged in any of several differentconfigurations. For example, if elements have a generally circular feedhorn, the elements may be arranged in a honeycomb configuration totightly pack the elements in a small amount of space. In some cases, theantenna elements are aligned in horizontal rows and vertical columns.

FIG. 21 is an example illustration of relative positions of receiveantenna 3 dB antenna contours associated with receive antenna elements406. The element 406 beam centers are numbered 1-16, with element 406 ₄identified by the number ‘4’ to the upper left of the beam centerindicator ‘x’. In some cases, there may be many more than 16 receiveantenna elements 406. However, for the sake of simplicity, only 16 areshown in FIG. 21 . A corresponding array of transmit antenna elements409 and their associated 3 dB antenna contours will look similar to FIG.21 . Therefore, for the sake of simplicity, only the array of receiveantenna elements 406 are shown. The area 2101 in the center is where allof the antenna element coverage areas overlap.

In some cases, at least one point within the relay coverage area (e.g.,satellite coverage area) falls within the 3 dB antenna contour of thecomponent beams of several antenna elements 406. In one such case, atleast one point is within the 3 dB antenna contour of at least 100different antenna elements 406. In another case, at least 10% of therelay coverage area lies within the 3 dB antenna contours of at least 30different antenna elements. In another case, at least 20% of the relaycoverage area lies within the 3 dB antenna contours of at least 20different antenna elements. In another case, at least 30% of the relaycoverage area lies within the 3 dB antenna contours of at least 10different antenna elements. In another case, at least 40% of the relaycoverage area lies within the 3 dB antenna contours of at least eightdifferent antenna elements. In another case, at least 50% of the relaycoverage area lies within the 3 dB antenna contours of at least fourdifferent antenna elements. However, in some cases, more than one ofthese relationships may be true.

In some cases, the end-to-end relay has a relay coverage area (e.g.,satellite coverage area) in which at least 25% of the points in theuplink relay coverage area are within (e.g., span) overlapping coverageareas of at least six receive antenna elements 406. In some cases, 25%of the points within the uplink relay coverage area are within (e.g.,span) overlapping coverage areas of at least four receive antennaelements 406. In some cases, the end-to-end relay has a coverage area inwhich at least 25% of the points in the downlink relay coverage area arewithin (e.g., span) overlapping coverage areas of at least six transmitantenna elements 409. In some cases, 25% of the points within thedownlink relay coverage area are within (e.g., span) overlappingcoverage areas of at least four transmit antenna elements 409.

In some cases, the receive antenna 402 may be pointed roughly at thesame coverage area as the transmit antenna 401, so that some receiveantenna element coverage areas may naturally correspond to particulartransmit antenna element coverage areas. In these cases, the receiveantenna elements 406 may be mapped to their corresponding transmitantenna elements 409 via the transponders 410, yielding similar transmitand receive antenna element coverage areas for each receive/transmitsignal path. In some cases, however, it may be advantageous to mapreceive antenna elements 406 to transmit antenna elements 409 that donot correspond to the same component beam coverage area. Accordingly,the mapping of the elements 406 of the receive antenna 402 to theelements 409 of the transmit antenna 401 may be randomly (or otherwise)permuted. Such permutation includes the case that results in the receiveantenna elements 406 not being mapped to the transmit antenna elements409 in the same relative location within the array or that have the samecoverage area. For example, each receive antenna element 406 within thereceive antenna element array may be associated with the sametransponder 410 as the transmit antenna element 409 located in themirror location of the transmit antenna element array. Any otherpermutation can be used to map the receive antenna elements 406 to thetransmit antenna elements 409 according to a permutation (e.g., paireach receive antenna element 406 with the same transponder to which anassociated transmit antenna element 409 is coupled in accordance with aparticular permutation of the receive antenna element 406 and thetransmit antenna element 409).

FIG. 22 is a table 4200 showing example mappings of receive antennaelements 406 to transmit antenna elements 409 through 16 transponders410. Each transponder 410 has an input that is exclusively coupled to anassociated receive antenna element 406 and an output that is exclusivelycoupled to an associated transmit antenna element 409 (e.g., there is aone to one relationship between each receive antenna element 406, onetransponder 410 and one transmit antenna element 409). In some cases,other receive antenna elements, transponders and transmit antennaelements may be present on the end-to-end relay (e.g., satellite) thatare not configured in a one to one relationship (and do not operate as apart of the end-to-end beamforming system).

The first column 4202 of the table 4200 identifies a transponder 410.The second column 4204 identifies a receive antenna element 406 to whichthe transponder 410 of the first column is coupled. The third column4206 of the table 4200 identifies an associated transmit antenna element409 to which the output of the transponder 410 is coupled. Each receiveantenna element 406 is coupled to the input of the transponder 410identified in the same row of the table 4200. Similarly, each transmitantenna element 409 is coupled to the output of the transponder 410identified in the same row of the table 4200. The third column of thetable 4200 shows an example of direct mapping in which each receiveantenna element 406 of the receive antenna array is coupled to the sametransponder 410 as a transmit antenna element 409 in the same relativelocation within the transmit antenna array. The fourth column 4208 oftable 4200 shows an example of interleaved mapping in which the firstreceive antenna element 406 is coupled to the first transponder 410 andto the tenth transmit antenna element 409. The second receive antennaelement 406 is coupled to the second transponder 410 and to the ninthtransmit antenna element 409, and so on. Some cases have otherpermutations, including a random mapping in which the particular pairingof the receive antenna element 406 and the transmit element 409 with atransponder 410 are randomly selected.

The direct mapping, which attempts to keep the transmit and receiveantenna element coverage areas as similar as possible for eachreceive/transmit signal path, generally yields the highest totalcapacity of the system. Random and interleaved permutations generallyproduce slightly less capacity but provide a more robust system in theface of AN outages, fiber outages in the terrestrial network, or loss ofreceive/transmit signal paths due to electronic failure on theend-to-end relay (e.g., in one or more transponders). Random andinterleaved permutations allow lower cost non-redundant ANs to be used.Random and interleaved permutations also provide less variation betweenthe capacity in the best performing beam and the capacity in the worstperforming beam. Random and interleaved permutations may also be moreuseful to initially operate the system with just a fraction of the ANsresulting in only a fraction of the total capacity being available butno loss in coverage area. An example of this is an incremental rolloutof ANs, where the system was initially operated with only 50% of the ANsdeployed. This may provide less than the full capacity, while stillallowing operation over the entire coverage area. As the demandincreases, more ANs can be deployed to increase the capacity until thefull capacity is achieved with all the ANs active. In some cases, achange in the composition of the ANs results in a re-calculation of thebeam weights. A change in composition may include changing the number orcharacteristics of one or more ANs. This may require a re-estimation ofthe end-to-end forward and/or return gains.

In some cases, the antenna is an array-fed reflector antenna with aparaboloid reflector. In other cases, the reflector does not have aparaboloid shape. An array of receive antenna elements 406 may bearranged to receive signals reflected by the reflector. Similarly, anarray of transmit antenna elements 409 may be arranged to form an arrayfor illuminating the reflector. One way to provide elements withoverlapping component beam antenna patterns is to have the elements 406,409 defocused (unfocused) as a consequence of the focal plane of thereflector being behind (or in front of) the array of elements 406, 409(i.e., the receive antenna array being located outside the focal planeof the receive reflector).

FIG. 23 is an illustration of a cross-section of a center-fed paraboloidreflector 1521. A focal point 1523 lies on a focal plane 1525 that isnormal to the central axis 1527 of the reflector 1521. Received signalsthat strike the reflector 1521 parallel to the central axis 1527 arefocused onto the focal point 1523. Likewise, signals that aretransmitted from an antenna element located at the focal point and thatstrike the reflector 1521 will be reflected in a focused beam from thereflector 1521 parallel to the central axis 1527. Such an arrangement isoften used in Single Feed per Beam systems to maximize the directivityof each beam and minimize overlap with beams formed by adjacent feeds.

FIG. 24 is an illustration of another paraboloid reflector 1621. Bylocating antenna elements 1629 (either receive antenna elements ortransmit antenna elements 406, 409, 3416, 3419, 3426, 3429) outside thefocal plane (e.g., in front of the focal plane 1625 of the reflector1621), the path of transmitted signals 1631 that strike the reflector1621 will not be parallel to one another as they reflect off thereflector 1621, resulting in a wider beam width than in the focusedcase. In some cases, reflectors that have shapes other than paraboloidsare used. Such reflectors may also result in defocusing the antenna. Theend-to-end beamforming system may use this type of defocused antenna tocreate overlap in the coverage area of adjacent antenna elements andthus provide a large number of useful receive/transmit paths for givenbeam locations in the relay coverage area.

In one case, a relay coverage area is established, in which 25% of thepoints within the relay coverage area are within the antenna elementcoverage areas of at least six component beam antenna patterns when theend-to-end relay is deployed (e.g., an end-to-end satellite relay is ina service orbit). Alternatively, 25% of the points within the relaycoverage area are within the antenna element coverage areas of at leastfour receive antenna elements. FIG. 25 is an illustration of an examplerelay coverage area (for an end-to-end satellite relay, also referred toas satellite coverage area) 3201 (shown with single cross-hatching) andthe area 3203 (shown with double cross-hatching) defined by the pointswithin the relay coverage area 3201 that are also contained within sixantenna element coverage areas 3205, 3207, 3209, 3211, 3213, 3215. Thecoverage area 3201 and the antenna element coverage areas 3205, 3207,3209, 3211, 3213, 3215 may be either receive antenna element coverageareas or transmit antenna element coverage areas and may be associatedwith only the forward link or only the return link. The size of theantenna element coverage areas 3205, 3207, 3209, 3211, 3213, 3215 isdetermined by the desired performance to be provided by the system. Asystem that is more tolerant of errors may have antenna element coverageareas that are larger than a system that is less tolerant. In somecases, each antenna element coverage area 3205, 3207, 3209, 3211, 3213,3215 is all points for which the component beam antenna gain is within10 dB of the peak component beam antenna gain for the antenna elementestablishing the component beam antenna pattern. In other cases, eachantenna element coverage area 3205, 3207, 3209, 3211, 3213, 3215 is allpoints for which the component beam antenna gain is within 6 dB of peakcomponent beam antenna gain. In still other cases, each antenna elementcoverage area 3205, 3207, 3209, 3211, 3213, 3215 is all points for whichthe component beam antenna gain is within 3 dB of peak component beamantenna gain. Even when an end-to-end relay has not yet been deployed(e.g., an end-to-end satellite relay is not in a service orbit, theend-to-end relay still has component beam antenna patterns that conformto the above definition. That is, antenna element coverage areascorresponding to an end-to-end relay in orbit can be calculated from thecomponent beam antenna patterns even when the end-to-end relay is not ina service orbit. The end-to-end relay may include additional antennaelements that do not contribute to beamforming and thus may not have theabove-recited characteristics.

FIG. 26 is an illustration of an end-to-end relay (e.g., satellite)antenna pattern 3300 in which all of the points within a relay coveragearea 3301 (e.g. satellite coverage area) are also contained within atleast four antenna element coverage areas 3303, 3305, 3307, 3309. Otherantenna elements may exist on the end-to-end relay and can have antennaelement coverage areas 3311 that contain less than all of the pointswithin the relay coverage area 3301.

The system may operate in any suitable spectrum. For example, anend-to-end beamforming system may operate in the C, L, S, X, V, Ka, Ku,or other suitable band or bands. In some such systems, the receive meansoperates in the C, L, S, X, V. Ka, Ku, or other suitable band or bands.In some cases, the forward uplink and the return uplink may operate inthe same frequency range (e.g., in vicinity of 30 GHz); and the returndownlink and the forward downlink may operate in a non-overlappingfrequency range (e.g., in the vicinity of 20 GHz). The end-to-end systemmay use any suitable bandwidth (e.g., 500 MHz, 1 GHz, 2 GHz, 3.5 GHz,etc.). In some cases, the forward and return links use the sametransponders.

To assist in system timing alignment, path lengths among the Ltransponders are set to match signal path time delays in some cases, forexample through appropriate cable length selection. The end-to-end relay(e.g., satellite) in some cases has a relay beacon generator 426 (e.g.satellite beacon) within a calibration support module 424 (see FIG. 15). The beacon generator 426 generates a relay beacon signal. Theend-to-end relay broadcasts the relay beacon signal to further aid insystem timing alignment as well as support feeder link calibration. Insome cases, the relay beacon signal is a pseudo-random (known as PN)sequence, such as a PN direct sequence spread spectrum signal that runsat a high chip rate (e.g., 100, 200, 400, or 800 million chips persecond (Mcps), or any other suitable value). In some cases, a linearlypolarized relay (e.g., satellite) beacon, receivable by both RHCP andLHCP antennas, is broadcast over a wide coverage area by an antenna,such as an antenna horn (not shown) or coupled into one or more of thetransponders 410 for transmission through the associated transmitantenna element 409. In an example system, beams are formed in multiple500 MHz bandwidth channels over the Ka band, and a 400 Mcps PN code isfiltered or pulse-shaped to fit within a 500 MHz bandwidth channel. Whenmultiple channels are used, the same PN code may be transmitted in eachof the channels. The system may employ one beacon for each channel, orone beacon for two or more channels.

Since there may be a large number of receive/transmit signal paths in anend-to-end relay, redundancy of individual receive/transmit signal pathsmay not be required. Upon failure of a receive/transmit signal path, thesystem may still perform very close to its previous performance level,although modification of beamforming coefficients may be used to accountfor the loss.

Ground Networks

The ground network of an example end-to-end beamforming system containsa number of geographically distributed Access Node (AN) Earth stationspointed at a common end-to-end relay. Looking first at the forward link,a Central Processing System (CPS) computes beam weights for transmissionof user data and interfaces to the ANs through a distribution network.The CPS also interfaces to the sources of data being provided to theuser terminals. The distribution network may be implemented in variousways, for example using a fiber optic cable infrastructure. Timingbetween the CPS and SANs may be deterministic (e.g., usingcircuit-switched channels) or non-deterministic (e.g., using apacket-switched network). In some cases, the CPS is implemented at asingle site, for example using custom application specific integratedcircuits (ASICs) to handle signal processing. In some cases, the CPS isimplemented in a distributed manner, for example using cloud computingtechniques.

Returning to the example of FIG. 5 , the CPS 505 may include a pluralityof feeder link modems 507. For the forward link, the feeder link modems507 each receive forward user data streams 509 from various datasources, such as the internet, a video headend (not shown), etc. Thereceived forward user data streams 509 are modulated by the modems 507into K forward beam signals 511. In some cases, K may be in the range of1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 or numbers in-between orgreater. Each of the K forward beam signals carries forward user datastreams to be transmitted on one of K forward user beams. Accordingly,if K=400, then there are 400 forward beam signals 511, each to betransmitted over an associated one of 400 forward user beams to aforward user beam coverage area 519. The K forward beam signals 511 arecoupled to a forward beamformer.

If M ANs 515 are present in the ground segment 502, then the output ofthe forward beamformer is M access node-specific forward signals 516,each comprising weighted forward beam signals corresponding to some orall of the K forward beam signals 511. The forward beamformer maygenerate the M access node-specific forward signals 516 based on amatrix product of the M×K forward beam weight matrix with the K forwarddata signals. A distribution network 518 distributes each of the Maccess node-specific forward signals to a corresponding one of the M ANs515. Each AN 515 transmits a forward uplink signal 521 comprising arespective access node-specific forward signal 516. Each AN 515transmits its respective forward uplink signal 521 for relay to one ormore (e.g., up to and including all) of the forward user beam coverageareas via one or more (e.g., up to and including all) of the forwardreceive/transmit signal paths of the end-to-end relay. Transponders 410,411 within the end-to-end relay 503 receive a composite input forwardsignal comprising a superposition 550 of forward uplink signals 521transmitted by a plurality (e.g., up to and including all) of the ANs515. Each transponder (e.g., each receive/transmit signal path throughthe relay) relays the composite input forward signal as a respectiveforward downlink signal to the user terminals 517 over the forwarddownlink.

FIG. 27 is an illustration of an example distribution of ANs 515. Eachof the smaller numbered circles represents the location of an AN 515.Each of the larger circles indicates a user beam coverage area 519. Insome cases, the ANs 515 are spaced approximately evenly over thecoverage area of the end-to-end relay 503. In other cases, the ANs 515may be distributed unevenly over the entire coverage area. In yet othercases, the ANs 515 may be distributed evenly or unevenly over one ormore sub-regions of the relay coverage area. Typically, systemperformance is best when the ANs 515 are uniformly distributed over theentire coverage area. However, considerations may dictate compromises inthe AN placement. For example, an AN 515 may be placed based on theamount of interference, rain, or other environmental conditions, cost ofreal estate, access to the distribution network, etc. For example, for asatellite-based end-to-end relay system that is sensitive to rain, moreof the ANs 515 may be placed in areas that are less likely to experiencerain-induced fading (e.g., the western United States). As anotherexample, ANs 515 may be placed more densely in high rain regions (e.g.,the southeastern United States) to provide some diversity gain tocounteract the effects of rain fading. ANs 515 may be located alongfiber routes to reduce distribution costs associated with the ANs 515.

The number of ANs 515, M, is a selectable parameter that can be selectedbased upon several criteria. Fewer ANs can result in a simpler, lowercost ground segment, and lower operational costs for the distributionnetwork. More ANs can result in larger system capacity. FIG. 28 shows asimulation of the normalized forward and return link capacity as afunction of the number of ANs deployed in an example system. Normalizedcapacity is the capacity with M ANs divided by the capacity obtainedwith the largest number of ANs in the simulation. The capacity increasesas the number of ANs increases, but it does not increase without bound.Both forward link and return link capacities approach an asymptoticlimit as the number of ANs is increased. This simulation was performedwith L=517 transmit and receive antenna elements and with the ANsdistributed uniformly over the coverage area, but this asymptoticbehavior of the capacity can be seen with other values for L and otherAN spatial distributions. Curves like those shown in FIG. 28 can behelpful in selection of the number of ANs, M, to be deployed and inunderstanding how the system capacity can be phased in as ANs areincrementally deployed, as discussed previously.

FIG. 29 is a block diagram of an example ground segment 502 for anend-to-end beamforming system. FIG. 29 may illustrate, for example,ground segment 502 of FIG. 5 . The ground segment 502 comprises CPS 505,distribution network 518, and ANs 515. CPS 505 comprises beam signalinterface 524, forward/return beamformer 513, distribution interface536, and beam weight generator 910.

For the forward link, beam signal interface 524 obtains forward beamsignals (FBS) 511 associated with each of the forward user beams. Beamsignal interface 524 may include forward beam data multiplexer 526 andforward beam data stream modulator 528. Forward beam data multiplexer526 may receive forward user data streams 509 comprising forward datafor transmission to user terminals 517. Forward user data streams 509may comprise, for example, data packets (e.g., TCP packets, UDP packets,etc.) for transmission to the user terminals 517 via the end-to-endbeamforming system 500 of FIG. 5 . Forward beam data multiplexer 526groups (e.g., multiplexes) the forward user data streams 509 accordingto their respective forward user beam coverage areas to obtain forwardbeam data streams 532. Forward beam data multiplexer 526 may use, forexample, time-domain multiplexing, frequency-domain multiplexing, or acombination of multiplexing techniques to generate forward beam datastreams 532. Forward beam data stream modulator 528 may modulate theforward beam data streams 532 according to one or more modulationschemes (e.g., mapping data bits to modulation symbols) to create theforward beam signals 511, which are passed to the forward/returnbeamformer 513. In some cases, the modulator 528 may frequency multiplexmultiple modulated signals to create a multi-carrier beam signal 511.Beam signal interface 524 may, for example, implement the functionalityof feeder link modems 507 discussed with reference to FIG. 5 .

Forward/return beamformer 513 may include forward beamformer 529 andreturn beamformer 531. Beam weight generator 910 generates an M×Kforward beam weight matrix 918. Techniques for generating the M×Kforward beam weight matrix 918 are discussed in more detail below.Forward beamformer 529 may include a matrix multiplier that calculates Maccess-node specific forward signals 516. For example, this calculationcan be based on a matrix product of the M×K forward beam weight matrix918 and a vector of the K forward beam signals 511. In some examples,each of the K forward beam signals 511 may be associated with one of Fforward frequency sub-bands. In this case, the forward beamformer 529may generate samples for the M access-node specific forward signals 516for each of the F forward frequency sub-bands (e.g., effectivelyimplementing the matrix product operation for each of the F sub-bandsfor respective subsets of the K forward beam signals 511. Distributioninterface 536 distributes (e.g., via distribution network 518) the Maccess node-specific forward signals 516 to the respective ANs 515.

For the return link, the distribution interface 536 obtains compositereturn signals 907 from ANs 515 (e.g., via distribution network 518).Each return data signal from user terminals 517 may be included inmultiple (e.g., up to and including all) of the composite return signals907. Beam weight generator 910 generates a K×M return beam weight matrix937. Techniques for generating the K×M return beam weight matrix 937 arediscussed in more detail below. Return beamformer 531 calculates Kreturn beam signals 915 for the K return user beam coverage areas. Forexample, this calculation can be based on a matrix product of the returnbeam weight matrix 937 and a vector of the respective composite returnsignals 907. Beam signal interface 524 may include return beam signaldemodulator 552 and return beam data de-multiplexer 554. Return beamsignal demodulator 552 may demodulate each of the return beam signals toobtain K return beam data streams 534 associated with the K return userbeam coverage areas. Return beam data de-multiplexer 554 mayde-multiplex each of the K return beam data streams 534 into respectivereturn user data streams 535 associated with the return data signalstransmitted from user terminals 517. In some examples, each of thereturn user beams may be associated with one of R return frequencysub-bands. In this case, the return beamformer 531 may generaterespective subsets of the return beam signals 915 associated with eachof the R return frequency sub-bands (e.g., effectively implementing thematrix product operation for each of the R return frequency sub-bands togenerate respective subsets of the return beam signals 915).

FIG. 30 is a block diagram of an example forward/return beamformer 513.The forward/return beamformer 513 comprises a forward beamformer 529, aforward timing module 945, a return beamformer 531, and a timing module947. The forward timing module 945 associates each of the M accessnode-specific forward signals 516 with a time stamp (e.g., multiplexesthe time stamp with the access node-specific forward signal in amultiplexed access node-specific forward signal) that indicates when thesignal is desired to arrive at the end-to-end relay. In this way, thedata of the K forward beam signals 511 that is split in a splittingmodule 904 within the forward beamformer 529 may be transmitted at theappropriate time by each of the ANs 515. The timing module 947 alignsthe receive signals based on time stamps. Samples of the M AN compositereturn signals (CRS) 907 are associated with time stamps indicating whenthe particular samples were transmitted from the end-to-end relay.Timing considerations and generation of the time stamps are discussed ingreater detail below.

The forward beamformer 529 has a data input 925, a beam weights input920 and an access node output 923. The forward beamformer 529 appliesthe values of an M×K beam weight matrix to each of the K forward datasignals 511 to generate M access node specific forward signals 521, eachhaving K weighted forward beam signals. The forward beamformer 529 mayinclude a splitting module 904 and M forward weighting and summingmodules 533. The splitting module 904 splits (e.g., duplicates) each ofthe K forward beam signals 511 into M groups 906 of K forward beamsignals, one group 906 for each of the M forward weighting and summingmodules 533. Accordingly, each forward weighting and summing module 533receives all K forward data signals 511.

A forward beam weight generator 917 generates an M×K forward beam weightmatrix 918. In some cases, the forward beam weight matrix 918 isgenerated based on a channel matrix in which the elements are estimatesof end-to-end forward gains for each of the K×M end-to-end forwardmultipath channels to form a forward channel matrix, as discussedfurther below. Estimates of the end-to-end forward gain are made in achannel estimator module 919. In some cases, the channel estimator has achannel data store 921 that stores data related to various parameters ofthe end-to-end multipath channels, as is discussed in further detailbelow. The channel estimator 919 outputs an estimated end-to-end gainsignal to allow the forward beam weight generator 917 to generate theforward beam weight matrix 918. Each of the weighting and summingmodules 533 are coupled to receive respective vectors of beamformingweights of the forward beam weight matrix 918 (only one such connectionis show in FIG. 30 for simplicity). The first weighting and summingmodule 533 applies a weight equal to the value of the 1,1 element of theM×K forward beam weight matrix 918 to the first of the K forward beamsignals 511 (discussed in more detail below). A weight equal to thevalue of the 1,2 element of the M×K forward beam weight matrix 918 isapplied to the second of the K forward beam signals 511. The otherweights of the matrix are applied in like fashion, on through the K^(th)forward beam signal 511, which is weighted with the value equal to the1, K element of the M×K forward beam weight matrix 918. Each of the Kweighted forward beam signals 903 are then summed and output from thefirst weighting and summing module 533 as an access node-specificforward signal 516. The access node-specific forward signal 516 outputby the first weighting and summing module 533 is then coupled to thetiming module 945. The timing module 945 outputs the accessnode-specific forward signal 516 to the first AN 515 through adistribution network 518 (see FIG. 5 ). Similarly, each of the otherweighting and summing modules 533 receive the K forward beam signals511, and weight and sum the K forward beam signals 511. The outputs fromeach of the M weighting and summing modules 533 are coupled through thedistribution network 518 to the associated M ANs 515 so that the outputfrom the m^(th) weighting and summing module is coupled to the m^(th) AN515. In some cases, jitter and uneven delay through the distributionnetwork, as well as some other timing considerations, are handled by thetiming module 945 by associating a time stamp with the data. Details ofan example timing technique are provided below with regard to FIGS. 36and 37 .

As a consequence of the beam weights applied by the forward beamformers529 at the ground segment 502, the signals that are transmitted from theANs 515 through the end-to-end relay 503 form user beams. The size andlocation of the beams that are able to be formed may be a function ofthe number of ANs 515 that are deployed, the number and antenna patternsof relay antenna elements that the signal passes through, the locationof the end-to-end relay 503, and/or the geographic spacing of the ANs515.

Referring now to the end-to-end return link 523 shown in FIG. 5 , a userterminal 517 within one of the user beam coverage areas 519 transmitssignals up to the end-to-end relay 503. The signals are then relayeddown to the ground segment 502. The signals are received by ANs 515.

Referring once again to FIG. 30 , M return downlink signals 527 arereceived by the M ANs 515 and are coupled, as composite return signals907, from the M ANs 515 through the distribution network 518 andreceived in an access node input 931 of the return beamformer 531.Timing module 947 aligns the composite return signals from the M ANs 515to each other and outputs the time-aligned signals to the returnbeamformer 531. A return beam weight generator 935 generates the returnbeam weights as a K×M return beam weight matrix 937 based on informationstored in a channel data store 941 within a channel estimator 943. Thereturn beamformer 531 has a beam weights input 939 through which thereturn beamformer 531 receives the return beam weight matrix 937. Eachof the M AN composite return signals 907 is coupled to an associated oneof M splitter and weighting modules 539 within the return beamformer531. Each splitter and weighting module 539 splits the time-alignedsignal into K copies 909. The splitter and weighting modules 539 weighteach of the K copies 909 using the k, m element of the K×M return beamweight matrix 937. Further details regarding the K×M return beam weightmatrix are provided below. Each set of K weighted composite returnsignals 911 is then coupled to a combining module 913. In some cases,the combining module 913 combines the k^(th) weighted composite returnsignal 911 output from each splitter and weighting module 539. Thereturn beamformer 531 has a return data signal output 933 that outputs Kreturn beam signals 915, each having the samples associated with one ofthe K return user beams 519 (e.g., the samples received through each ofthe M ANs). Each of the K return beam signals 915 may have samples fromone or more user terminals 517. The K combined and aligned, beamformedreturn beam signals 915 are coupled to the feeder link modems 507 (seeFIG. 5 ). Note that the return timing adjustment may be performed afterthe splitting and weighting. Similarly, for the forward link, theforward timing adjustment may be performed before the beamforming.

As discussed above, forward beamformer 529 may perform matrix productoperations on input samples of K forward beam signals 511 to calculate Maccess node-specific forward signal 516 in real-time. As the beambandwidth increases (e.g., to support shorter symbol duration) and/or Kand M become large, the matrix product operation becomes computationallyintensive and may exceed the capabilities of a single computing node(e.g., a single computing server, etc.). The operations of returnbeamformer 531 are similarly computationally intensive. Variousapproaches may be used to partition computing resources of multiplecomputing nodes in the forward/return beamformer 513. In one example,the forward beamformer 529 of FIG. 30 may be partitioned into separateweighting and summing modules 533 for each of the M ANs 515, which maybe distributed into different computing nodes. Generally, theconsiderations for implementations include cost, power consumption,scalability relative to K, M, and bandwidth, system availability (e.g.,due to node failure, etc.), upgradeability, and system latency. Theexample above is per row (or column). Vice versa is possible. Othermanners of grouping the matrix operations may be considered (e.g., splitinto four with [1,1 to K/2, M/2], [ . . . ], computed individually andsummed up).

In some cases, the forward/return beamformer 513 may include atime-domain multiplexing architecture for processing of beam weightingoperations by time-slice beamformers. FIG. 31 is a block diagram of anexample forward beamformer 529 comprising multiple forward time-slicebeamformers with time-domain de-multiplexing and multiplexing. Theforward beamformer 529 includes a forward beam signal de-multiplexer3002, N forward time-slice beamformers 3006, and a forward access nodesignal multiplexer 3010.

Forward beam signal de-multiplexer 3002 receives forward beam signals511 and de-multiplexes the K forward beam signals 511 into forward timeslice inputs 3004 for input to the N forward time-slice beamformers3006. For example, the forward beam signal de-multiplexer 3002 sends afirst time-domain subset of samples for the K forward beam signals 511to a first forward time-slice beamformer 3006, which generates samplesassociated with the M access node-specific forward signals correspondingto the first time-domain subset of samples. The forward time-slicebeamformer 3006 outputs the samples associated with the M accessnode-specific forward signals for the first time-domain subset ofsamples via its forward time slice output 3008 to the forward accessnode signal multiplexer 3010. The forward time-slice beamformer 3006 mayoutput the samples associated with each of the M access node-specificforward signals with synchronization timing information (e.g., thecorresponding time-slice index, etc.) used by the access nodes to cause(e.g., by pre-correcting) the respective access node-specific forwardsignals to be synchronized when received by the end-to-end relay. Theforward access node signal multiplexer 3010 multiplexes time-domainsubsets of samples for the M access node-specific forward signalsreceived via the N forward time slice outputs 3008 to generate the Maccess node-specific forward signals 516. Each of the forward time-slicebeamformers 3006 may include a data buffer, a beam matrix buffer, andbeam weight processor implementing the matrix product operation. Thatis, each of the forward time-slice beamformers 3006 may implementcomputations mathematically equivalent to the splitting module 904 andforward weighting and summing modules 533 shown for forward beamformer529 of FIG. 30 during processing of the samples of one time slice-index.Updating of the beam weight matrix may be performed incrementally. Forexample, the beam weight matrix buffers for forward time-slicebeamformers may be updated during idle time in a rotation of time-sliceindices t through the N forward time-slice beamformers 3006.Alternatively, each forward time-slice beamformer may have two buffersthat can be used in a ping-pong configuration (e.g., one can be updatedwhile the other is being used). In some cases, multiple buffers can beused to store beam weights corresponding to multiple user beam patterns(e.g., multiple user coverage areas). Beam weight buffers and databuffers for forward time-slice beamformers 3006 may be implemented asany type of memory or storage including dynamic or static random accessmemory (RAM). Beam weight processing may be implemented in anapplication specific integrated circuit (ASIC) and/or a fieldprogrammable gate array (FPGA), and may include one or more processingcores (e.g., in a cloud computing environment). Additionally oralternatively, the beam weight buffer, data buffer, and beam weightprocessor may be integrated within one component.

FIG. 32 illustrates a simplified example ground segment showing theoperation of a forward time-slice beamformer 529. In the example of FIG.32 , forward beamformer 529 receives four forward beam signals (e.g.,K=4), generates access node-specific forward signals for five ANs (e.g.,M=5), and has three forward time-slice beamformers (e.g., N=3). Theforward beam signals are denoted by FBk:t, where k is the forward beamsignal index and t is the time-slice index (e.g., corresponding to atime-domain subset of samples). The forward beam signal de-multiplexer3002 receives four time-domain subsets of samples of the forward beamsignals associated with four forward user beams and de-multiplexes eachforward beam signal so that one forward time slice input 3004 includes,for a particular time-slice index t, the time-domain subsets of samplesfrom each of the forward beam signals 511. For example, time-domainsubsets can be a single sample, a contiguous block of samples, or adiscontiguous (e.g., interleaved) block of samples as described below.The forward time-slice beamformers 3006 generate (e.g., based on theforward beam signals 511 and forward beam weight matrix 918) each of theM access-node specific forward signals for the time-slice index t,denoted by AFm:t. For example, the time-domain subsets of samples FB1:0,FB2:0, FB3:0, and FB4:0 for time-slice index t=0 are input to the firstforward time-slice beam former TSBF[1] 3006, which generatescorresponding samples of access node-specific forward signals AF1:0,AF2:0, AF3:0, AF4:0, and AF5:0 at a forward time slice output 3008. Forsubsequent time-slice index values t=1, 2, the time-domain subsets ofsamples of forward beam signals 511 are de-multiplexed by the forwardbeam signal de-multiplexer 3002 for input to second and third forwardtime-slice beamformers 3006, which generate access node-specific forwardsignals associated with the corresponding time-slice indices t atforward time slice outputs 3008. FIG. 32 also shows that at time-sliceindex value t=3, the first forward time-slice beamformer generatesaccess node-specific forward signals associated with the correspondingtime-slice index 3. The matrix product operation performed by eachforward time-slice beamformer 3006 for one time-slice index value t maytake longer than the real time of the time-domain subset of samples(e.g., the number of samples S multiplied by the sample rate t_(S)).However, each forward time-slice beamformer 3006 may only process onetime-domain subset of samples every N time-slice indices t. Forwardaccess node signal multiplexer 3010 receives forward time slice outputs3030 from each of the forward time-slice beamformers 3006 andmultiplexes the time-domain subsets of samples to generate the M accessnode-specific forward signals 516 for distribution to respective ANs.

FIG. 33 is a block diagram of an example return beamformer 531comprising multiple return time-slice beamformers with time-domainde-multiplexing and multiplexing. The return beamformer 531 includes areturn composite signal de-multiplexer 3012, N return time-slicebeamformers 3016, and a return beam signal multiplexer 3020. Returncomposite signal de-multiplexer 3012 receives M composite return signals907 (e.g., from M ANs) and de-multiplexes the M composite return signals907 into return time slice inputs 3014 for input to the N returntime-slice beamformers 3016. Each of the return time-slice beamformers3016 output the samples associated with the K return beam signals 915for corresponding time-domain subsets of samples via respective returntime slice outputs 3018 to the return beam signal multiplexer 3020. Thereturn beam signal multiplexer 3020 multiplexes the time-domain subsetsof samples for the K return beam signals received via the N return timeslice outputs 3018 to generate the K return beam signals 915. Each ofthe return time-slice beamformers 3016 may include a data buffer, a beammatrix buffer, and beam weight processor implementing the matrix productoperation. That is, each of the return time-slice beamformers 3016 mayimplement computations mathematically equivalent to the splitter andweighting modules 539 and combining module 913 shown for returnbeamformer 531 of FIG. 30 during processing of the samples of one timeslice-index. As discussed above with the forward time-slice beamformers,updating of the beam weight matrix may be performed incrementally usinga ping-pong beam weight buffer configuration (e.g., one can be updatedwhile the other is being used). In some cases, multiple buffers can beused to store beam weights corresponding to multiple user beam patterns(e.g., multiple user coverage areas). Beam weight buffers and databuffers for return time-slice beamformers 3016 may be implemented as anytype of memory or storage including dynamic or static random accessmemory (RAM). Beam weight processing may be implemented in anapplication specific integrated circuit (ASIC) and/or a fieldprogrammable gate array (FPGA) and may include one or more processingcores. Additionally or alternatively, the beam weight buffer, databuffer, and beam weight processor may be integrated within onecomponent.

FIG. 34 illustrates a simplified example ground segment showing theoperation of a return beamformer 531 employing time-domain multiplexing.In the example of FIG. 33 , return beamformer 531 receives fivecomposite return signals (e.g., M=5), generates return beam signals forfour return user beams (e.g., K=5), and has three time-slice beamformers(e.g., N=3). The composite return signals are denoted by RCm:t, where mis the AN index and t is the time-slice index (e.g., corresponding to atime-domain subset of samples). The return composite signalde-multiplexer 3012 receives four time-domain subsets of samples of thecomposite return signals from five ANs and de-multiplexes each compositereturn signal so that one return time slice input 3014 includes, for aparticular time-slice index t, the corresponding time-domain subsets ofsamples from each of the composite return signals 907. For example,time-domain subsets can be a single sample, a contiguous block ofsamples, or a discontiguous (e.g., interleaved) block of samples asdescribed below. The return time-slice beamformers 3016 generate (e.g.,based on the composite return signals 907 and return beam weight matrix937) each of the K return beam signals for the time-slice index t,denoted by RBk:t. For example, the time-domain subsets of samples RC1:0,RC2:0, RC3:0, RC4:0, and RC5:0 for time-slice index t=0 are input to afirst return time-slice beam former 3016, which generates correspondingsamples of return beam signals RB1:0, RB2:0, RB3:0, and RB4:0 at areturn time slice output 3018. For subsequent time-slice index valuest=1, 2, the time-domain subsets of samples of composite return signals907 are de-multiplexed by the return composite signal de-multiplexer3012 for input to a second and a third return time-slice beamformer3016, respectively, which generate samples for the return beam signalsassociated with the corresponding time-slice indices t at return timeslice outputs 3018. FIG. 34 also shows that at time-slice index valuet=3, the first return time-slice beamformer generates samples of returnbeam signals associated with the corresponding time-slice index 3. Thematrix product operation performed by each return time-slice beamformer3016 for one time-slice index value t may take longer than the real timeof the time-domain subset of samples (e.g., the number of samples Smultiplied by the sample rate t_(S)). However, each return time-slicebeamformer 3016 may only process one time-domain subset of samples everyN time-slice indices t. Return beam signal multiplexer 3020 receivesreturn time slice outputs 3018 from each of the return time-slicebeamformers 3016 and multiplexes the time-domain subsets of samples togenerate the K return beam signals 915.

Although FIGS. 31-34 illustrate the same number N of forward time-slicebeamformers 3006 as return time-slice beamformers 3016, someimplementations may have more or fewer forward time-slice beamformers3006 than return time-slice beamformers 3016. In some examples, forwardbeamformer 529 and/or return beamformer 531 may have spare capacity forrobustness to node failure. For example, if each forward time-slicebeamformer 3006 takes t_(FTS) to process one set of samples for atime-slice index t having a real-time time-slice duration ID, wheret_(FTS)=N·t_(D), the forward beamformer 529 may have N+E forwardtime-slice beamformers 3006. In some examples, each of the N+E forwardtime-slice beamformers 3006 are used in operation, with each forwardtime-slice beamformer 3006 having an effective extra capacity of E/N. Ifone forward time-slice beamformer 3006 fails, the operations may beshifted to another forward time-slice beamformer 3006 (e.g., byadjusting how time-domain samples (or groups of samples) are routedthrough the time-domain de-multiplexing and multiplexing). Thus, forwardbeamformer 529 may be tolerant of up to E forward time-slice beamformers3006 failing before system performance is impacted. In addition, extracapacity allows for system maintenance and upgrading of time-slicebeamformers while the system is operating. For example, upgrading oftime-slice beamformers may be performed incrementally because the systemis tolerant of different performance between time-slice beamformers. Thedata samples associated with a time-slice index t may be interleaved.For example, a first time-slice index to may be associated with samples0, P, 2P, . . . (S−1)*P, while a second time-slice index t, may beassociated with samples 1, P+1, 2P+1 . . . (S−1)*P+1, etc., where S isthe number of samples in each set of samples, and P is the interleavingduration. The interleaving may also make the system more robust totime-slice beamformer failures, because each time-slice beamformer blockof samples are separated in time such that errors due to a missing blockwould be distributed in time, similarly to the advantage frominterleaving in forward error correction. In fact, the distributederrors caused by time-slice beamformer failure may cause effects similarto noise and not result in any errors to user data, especially ifforward error coding is employed. Although examples where N=3 have beenillustrated, other values of N may be used, and N need not have anyparticular relationship to K or M.

As discussed above, forward beamformer 529 and return beamformer 531illustrated in FIGS. 31 and 33 , respectively, may perform time-domainde-multiplexing and multiplexing for time-slice beamforming for onechannel or frequency sub-band. Multiple sub-bands may be processedindependently using an additional sub-band mux/demux switching layer.FIG. 35 is a block diagram of an example multi-band forward/returnbeamformer 513 that employs sub-band de-multiplexing and multiplexing.The multi-band forward/return beamformer 513 may support F forwardsub-bands and R return sub-bands.

Multi-band forward/return beamformer 513 includes F forward sub-bandbeamformers 3026, R return sub-band beamformers 3036, and a sub-bandmultiplexer/de-multiplexer 3030. For example, the forward beam signals511 may be split up into F forward sub-bands. Each of the F forwardsub-bands may be associated with a subset of the K forward user beamcoverage areas. That is, the K forward user beam coverage areas mayinclude multiple subsets of forward user beam coverage areas associatedwith different (e.g., different frequency and/or polarization, etc.)frequency sub-bands, where the forward user beam coverage areas withineach of the subsets may be non-overlapping (e.g., at 3 dB signalcontours, etc.). Thus, each of the forward sub-band beamformer inputs3024 may include a subset K₁ of the forward beam signals 511. Each ofthe F forward beamformers 3026 may include the functionality of forwardbeamformer 529, generating forward sub-band beamformer outputs 3028 thatcomprise the M access node-specific forward signals associated with thesubset of the forward beam signals 511 (e.g., a matrix product of the K₁forward beam signals with an M×K₁ forward beam weight matrix). Thus,each of the ANs 515 may receive multiple access node-specific forwardsignals associated with different frequency sub-bands (e.g., for each ofthe F forward sub-bands). The ANs may combine (e.g., sum) the signals indifferent sub-bands in the forward uplink signals, as discussed in moredetail below. Similarly, ANs 515 may generate multiple composite returnsignals 907 for R different return sub-bands. Each of the R returnsub-bands may be associated with a subset of the K return user beamcoverage areas. That is, the K return user beam coverage areas mayinclude multiple subsets of return user beam coverage areas associatedwith different frequency sub-bands, where the return user beam coverageareas within each of the subsets may be non-overlapping (e.g., at 3 dBsignal contours, etc.). The sub-band multiplexer/de-multiplexer 3030 maysplit the composite return signals 907 into the R return sub-bandbeamformer inputs 3034. Each of the return sub-band beamformers 3036 maythen generate a return sub-band beamformer output 3038, which mayinclude the return beam signals 915 for a subset of the return userbeams (e.g., to the feeder link modems 507 or return beam signaldemodulator, etc.). In some examples, the multi-band forward/returnbeamformer 513 may support multiple polarizations (e.g., right-handcircular polarization (RHCP), left-hand circular polarization (LHCP),etc.), which in some cases may effectively double the number ofsub-bands.

In some cases, time-slice multiplexing and de-multiplexing for forwardbeamformer 529 and return beamformer 531 (e.g., beam signalde-multiplexer 3002, forward access node signal multiplexer 3010, returncomposite signal de-multiplexer 3012, return beam signal multiplexer3020) and sub-band multiplexing/de-mutiplexing (sub-bandmultiplexer/de-multiplexer 3030) may be performed by packet switching(e.g., Ethernet switching, etc.). In some cases, the time-slice andsub-band switching may be performed in the same switching nodes, or in adifferent order. For example, a fabric switching architecture may beused where each switch fabric node may be coupled with a subset of theANs 515, forward time-slice beamformers 3006, return time-slicebeamformers 3016, or feeder link modems 507. A fabric switchingarchitecture may allow, for example, any AN to connect (e.g., viaswitches and/or a switch fabric interconnect) to any forward time-slicebeamformer or return time-slice beamformer in a low-latency,hierarchically flat architecture. In one example, a system supportingK≤600, M≤600, and a 500 MHz bandwidth (e.g., per sub-band) with fourteensub-bands for the forward or return links may be implemented by acommercially available interconnect switch platform with 2048 10 GigEports.

Delay Equalization

In some cases, differences in the propagation delays on each of thepaths between the end-to-end relay 503 and the CPS 505 areinsignificant. For example, on the return link, when the same signal(e.g., data to or from a particular user) is received by multiple ANs515, each instance of the signal may arrive at the CPS essentiallyaligned with each other instance of the signal. Likewise, when the samesignal is transmitted to a user terminal 517 through several ANs 515,each instance of the signal may arrive at the user terminal 517essentially aligned with each other instance of the signal. In otherwords, signals may be phase and time aligned with sufficient precisionthat signals will coherently combine, such that the path delays andbeamforming effects are small relative to the transmitted symbol rate.As an illustrative example, if the difference in path delays is 10microseconds, the beamforming bandwidth can be on the order of tens ofkHz and one can use a narrow bandwidth signal, say ≈10 ksps with a smallpossible degradation in performance. The 10 ksps signaling rate has asymbol duration of 100 microseconds and the 10 microsecond delay spreadis only one tenth of the symbol duration. In these cases, for thepurposes of the system analysis, it may be assumed that signals receivedby the end-to-end relay at one instant will be relayed and transmittedat essentially the same time, as described earlier.

In other cases, there may be a significant difference in the propagationdelay relative to the signaling interval (transmitted symbol duration)of the signals transmitted from the transmit antenna elements 409 to theANs 515. The path that the signals take from each AN 515 through thedistribution network 518 may contain significant delay variations. Inthese cases, delay equalization may be employed to match the pathdelays.

For end-to-end return link signals received through the distributionnetwork 518 by the CPS 505, signals may be time aligned by using a relaybeacon signal transmitted from the end-to-end relay, for example a PNbeacon as described earlier. Each AN 515 may time stamp the compositereturn signal using the relay beacon signal as a reference. Therefore,different ANs 515 may receive the same signal at different times, butthe received signals in each AN 515 may be time stamped to allow the CPS505 to time align them. The CPS 505 may buffer the signals so thatbeamforming is done by combining signals that have the same time stamp.

Returning to FIGS. 33 and 34 , delay equalization for the return linkmay be performed by de-multiplexing the composite return signals to thereturn time-slice beamformers 3016. For example, each AN may split upthe composite return signal into sets of samples associated withtime-slice indices t, which may include interleaved samples of thecomposite return signal. The time-slice indices t may be determinedbased on the relay beacon signal. The ANs may send the subsets ofsamples multiplexed with the corresponding time-slice indices t (e.g.,as a multiplexed composite return signal) to the return beamformer 531,which may serve as synchronization timing information on the returnlink. The subsets of samples from each AN may be de-multiplexed (e.g.,via switching) and one return time-slice beamformer 3016 may receive thesubsets of samples from each AN for a time-slice index t (for one ofmultiple sub-bands, in some cases). By performing the matrix product ofthe return beam weight matrix and the subsets of samples from each ofthe M composite return signals associated with the time-slice index t,return time-slice beamformer 3016 may align the signals relayed by theend-to-end relay at the same time for applying the return beam weightmatrix.

For the forward link, the beamformer 513 within the CPS 505 may generatea time stamp that indicates when each access node-specific forwardsignal transmitted by the ANs 515 is desired to arrive at the end-to-endrelay 503. Each AN 515 may transmit an access node beacon signal 2530,for example a loopback PN signal. Each such signal may be looped-backand transmitted back to the ANs 515 by the end-to-end relay 503. The ANs515 may receive both the relay beacon signal and the relayed(looped-back) access node beacon signals from any or all of the ANs. Thereceived timing of the access node beacon signal relative to receivetiming of the relay beacon signal indicates when the access node beaconsignal arrived at the end-to-end relay. Adjusting the timing of theaccess node beacon signal such that, after relay by the end-to-endrelay, it arrives at the AN at the same time as the relay beacon signalarrives at the AN, forces the access node beacon signal to arrive at theend-to-end relay synchronized with the relay beacon. Having all ANsperform this function enables all access node beacon signals to arriveat the end-to-end relay synchronized with the relay beacon. The finalstep in the process is to have each AN transmit its access node-specificforward signals synchronized with its access node beacon signal. Thiscan be done using timestamps as described subsequently. Alternatively,the CPS may manage delay equalization by sending the respective accessnode-specific forward signals offset by the respective time-domainoffsets to the ANs (e.g., where the timing via the distribution networkis deterministic). In some cases, the feeder-link frequency range may bedifferent from the user-link frequency range. When the feeder-linkdownlink frequency range (e.g., a frequency range in V band) isnon-overlapping with the user-link downlink frequency range (e.g., afrequency range in Ka band), and the ANs are within the user coveragearea, the ANs may include antennas and receivers operable over theuser-link downlink frequency range in order to receive the relayedaccess node beacon signals via the receive/transmit signal paths of theend-to-end relay. In such a case, the end-to-end relay can include afirst relay beacon generator that generates a first relay beacon signalin the user-link downlink frequency range to support feeder linksynchronization. The end-to-end relay can also include a second relaybeacon generator that generates a second relay beacon signal in thefeeder-link downlink frequency range to support removal of feeder-linkimpairments from the return downlink signals.

FIG. 36 is an illustration of PN sequences used to align the timing ofthe system. The horizontal axis of the figure represents time. An AN₁ PNsequence 2301 of chips 2303 is transmitted in the access node beaconsignal from the first AN. The relative time of arrival of this sequenceat the end-to-end relay is depicted by the PN sequence 2305. There is atime shift of PN sequence 2305 with respect to AN₁ PN sequence 2301, dueto the propagation delay from the AN to the end-to-end relay. A relay PNbeacon sequence 2307 is generated within, and transmitted from, theend-to-end relay in a relay beacon signal. A PN chip of the relay PNbeacon sequence 2307 at time T₀ 2315 is aligned with a PN chip 2316 ofthe AN₁ PN received signal 2305 at time T₀. The PN chip 2316 of the AN₁PN received signal 2305 is aligned with the PN chip 2315 of the relay PNbeacon 2307 when the AN₁ transmit timing is adjusted by the properamount. The PN sequence 2305 is looped back from the end-to-end relayand the PN sequence 2317 is received at AN₁. A PN sequence 2319transmitted from the end-to-end relay in the relay PN beacon is receivedat AN₁. Note that the PN sequences 2317, 2319 are aligned at AN₁indicating that they were aligned at the end-to-end relay.

FIG. 37 shows an example of an AN₂ that has not properly adjusted thetiming of the PN sequence generated in the AN₂. Notice that the PNsequence 2311 generated by the AN₂ is received at the end-to-end relayshown as sequence 2309 with an offset by an amount dt from the relay PNbeacon PN sequence 2307. This offset is due to an error of the timingused to generate the sequence in the AN₂. Also, note that the arrival ofthe AN₂ PN sequence 2321 at AN₂ is offset from the arrival of the relayPN beacon PN sequence at AN₂ 2323 by the same amount dt. The signalprocessing in AN₂ will observe this error and may make a correction tothe transmit timing by adjusting the timing by an amount dt to align thePN sequences 2321, 2323.

In FIGS. 36 and 37 the same PN chip rate has been used for the relay PNbeacon and all of the AN (loopback) PN signals for ease of illustrationof the concept. The same timing concepts can be applied with differentPN chip rates. Returning to FIGS. 31 and 32 , the time-slice indices tmay be used for synchronizing the access node-specific forward signalsreceived from each of the ANs at the end-to-end relay. For example, thetime-slice indices t may be multiplexed with the access node-specificforward signals 516. Each AN may transmit samples of the accessnode-specific forward signals with a particular time-slice index taligned with corresponding timing information in the PN sequence ofchips transmitted in the respective access node beacon signals. Becausethe respective access node beacon signals have been adjusted tocompensate for the respective path delays and phase shifts between theANs and the end-to-end relay, the samples associated with the time-sliceindex t will arrive at the end-to-end relay with timing synchronized andphase aligned correctly relative to each other.

In cases where ANs receive their own access node beacon signals, it ispossible to loop back the access node beacon signals using the sameend-to-end relay communication hardware that is also carrying theforward direction communication data. In these cases, the relative gainsand/or phases of the transponders in the end-to-end relay can beadjusted as subsequently described.

FIG. 38 is a block diagram of an example AN 515. The AN 515 comprisesreceiver 4002, receive timing and phase adjuster 4024, relay beaconsignal demodulator 2511, multiplexer 4004, network interface 4006,controller 2523, de-multiplexer 4060, transmit timing and phasecompensator 4020, and transmitter 4012. Network interface 4006 may beconnected to, for example, CPS 505 via network port 4008.

On the return link, receiver 4002 receives a return downlink signal 527.The return downlink signal 527 may include, for example, a composite ofreturn uplink signals relayed by the end-to-end relay (e.g., viamultiple receive/transmit signal paths, etc.) and the relay beaconsignal. Receiver 4002 may perform, for example, down-conversion andsampling. Relay beacon signal demodulator 2511 may demodulate the relaybeacon signal in the digitized composite return signal 907 to obtainrelay timing information 2520. For example, relay beacon signaldemodulator 2511 may perform demodulation to recover the chip timingassociated with the relay PN code and generate time stamps correspondingto the transmission time from the end-to-end relay for samples of thedigitized composite return signal 527. Multiplexer 4004 may multiplexthe relay timing information 2520 with the samples of the digitizedcomposite return signal (e.g., to form a multiplexed composite returnsignal) to be sent to the CPS 505 (e.g., via network interface 4006).Multiplexing the relay timing information 2520 may include generatingsubsets of samples corresponding to time-slice indices t for sending tothe CPS 505. For example, multiplexer 4004 may output subsets of samplesassociated with each time slice index t for input to the returntime-slice beamforming architecture described above with reference toFIGS. 33, 34, and 35 . Multiplexer 4004 may include an interleaver 4044for interleaving samples for each subset of samples, in some cases.

On the forward link, network interface 4006 may obtain AN input signal4014 (e.g., via network port 4008). De-multiplexer 4060 may de-multiplexAN input signal 4014 to obtain access node-specific forward signal 516and forward signal transmit timing information 4016 indicatingtransmission timing for the access node-specific forward signal 516. Forexample, the access node-specific forward signal 516 may comprise theforward signal transmit timing information (e.g., multiplexed with datasamples, etc.). In one example, the access node-specific forward signal516 comprises sets of samples (e.g., in data packets), where each set ofsamples is associated with a time-slice index t. For example, each setof samples may be samples of the access node-specific forward signal 516generated according to the forward time-slice beamforming architecturediscussed above with reference to FIGS. 31, 32 and 35 . De-multiplexer4060 may include a de-interleaver 4050 for de-interleaving samplesassociated with time-slice indices t.

Transmit timing and phase compensator 4020 may receive and buffer accessnode-specific forward signal 516 and output forward uplink signalsamples 4022 for transmission by the transmitter 4012 at an appropriatetime as forward uplink signal 521. The transmitter 4012 may performdigital-to-analog conversion and up-conversion to output the forwarduplink signal 521. Forward uplink signal samples 4022 may include theaccess node-specific forward signal 516 and an access node beacon signal2530 (e.g., loopback PN signal), which may include transmit timinginformation (e.g., PN code chip timing information, frame timinginformation, etc.). Transmit timing and phase compensator 4020 maymultiplex the access node-specific forward signal 516 with the accessnode beacon signal 2530 such that the forward signal transmit timing andphase information 4016 is synchronized to corresponding transmit timingand phase information in the access node beacon signal 2530.

In some examples, generation of the access node beacon signal 2530 isperformed locally at the AN 515 (e.g., in access node beacon signalgenerator 2529). Alternatively, generation of the access node beaconsignal 2530 may be performed in a separate component (e.g., CPS 505) andsent to the AN 515 (e.g., via network interface 4006). As discussedabove, the access node beacon signal 2530 may be used to compensate theforward uplink signal 521 for path differences and phase shifts betweenthe AN and the end-to-end relay. For example, the access node beaconsignal 2530 may be transmitted in the forward uplink signal 521 andrelayed by the end-to-end relay to be received back at receiver 4002.The controller 2523 may compare relayed transmit timing and phaseinformation 4026 obtained (e.g., by demodulation, etc.) from the relayedaccess node beacon signal with receive timing and phase information 4028obtained (e.g., by demodulation, etc.) from the relay beacon signal. Thecontroller 2523 may generate a timing and phase adjustment 2524 forinput to the transmit timing and phase compensator 4020 to adjust theaccess node beacon signal 2530 to compensate for the path delay andphase shifts. For example, the access node beacon signal 2530 mayinclude a PN code and frame timing information (e.g., one or more bitsof a frame number, etc.). The transmit timing and phase compensator 4020may, for example, adjust the frame timing information for coarsecompensation for the path delay (e.g., output frame timing informationin the access node beacon signal such that the relayed access nodebeacon signal will have the relayed transmit frame timing informationcoarsely aligned with corresponding frame timing information in therelay beacon signal, changing which chip of the PN code is considered tobe the LSB, etc.). Additionally or alternatively, the transmit timingand phase compensator 4020 may perform timing and phase adjustments tothe forward uplink signal samples 4022 to compensate for timing or phasedifferences between the relayed transmit timing and phase information4026 and receive timing and phase information 4028. For example, wherethe access node beacon signal 2530 is generated based on a localoscillator, timing or phase differences between the local oscillator andthe received relay beacon signal may be corrected by timing and phaseadjustments to the forward uplink signal samples 4022. In some examples,demodulation of the access node beacon signal is performed locally atthe AN 515 (e.g., in access node beacon signal demodulator 2519).Alternatively, demodulation of the access node beacon signal may beperformed in a separate component (e.g., CPS 505) and the relayedtransmit timing and phase information 4026 may be obtained in othersignaling (e.g., via network interface 4006). For example, deep fadingmay make reception and demodulation of the AN's own relayed access nodebeacon signal difficult without transmission at higher power than othersignaling, which may reduce the power budget for communication signals.Thus, combining reception of the relayed access node beacon signal frommultiple ANs 515 may increase the effective received power anddemodulation accuracy for the relayed access node beacon signal. Thus,demodulation of the access node beacon signal from a single AN 515 maybe performed using downlink signals received at multiple ANs 515.Demodulation of the access node beacon signal may be performed at theCPS 505 based on the composite return signals 907, which may alsoinclude signal information for the access node beacon signals from mostor all ANs 515. If desired, end-to-end beamforming for the access nodebeacon signals can be performed taking into account the access nodebeacon uplinks (e.g., C_(r)), relay loopback (e.g., E), and/or accessnode beacon downlinks (e.g., C_(t)).

Feeder Link Impairment Removal

In addition to delay equalization of the signal paths to the end-to-endrelay from all the ANs, the phase shifts induced by feeder links can beremoved prior to beamforming. The phase shift of each of the linksbetween the end-to-end relay and the M ANs will be different. The causesfor different phase shifts for each link include, but are not limitedto, the propagation path length, atmospheric conditions such asscintillation, Doppler frequency shift, and different AN oscillatorerrors. These phase shifts are generally different for each AN and aretime varying (due to scintillation, Doppler shift, and difference in theAN oscillator errors). By removing dynamic feeder link impairments, therate at which beam weights adapt may be slower than an alternative wherethe beam weights adapt fast enough to track the dynamics of the feederlink.

In the return direction, feeder downlink impairments to an AN are commonto both the relay PN beacon and user data signals (e.g., return downlinksignals). In some cases, coherent demodulation of the relay PN beaconprovides channel information that is used to remove most or all of theseimpairments from the return data signal. In some cases, the relay PNbeacon signal is a known PN sequence that is continually transmitted andlocated in-band with the communications data. The equivalent (oreffective) isotropically radiated power (EIRP) of this in-band PN signalis set such that the interference to the communications data is notlarger than a maximum acceptable level. In some cases, a feeder linkimpairment removal process for the return link involves coherentdemodulation and tracking of the received timing and phase of the relayPN beacon signal. For example, relay beacon signal demodulator 2511 maydetermine receive timing and phase adjustments 2512 to compensate forfeeder link impairment based on comparing the relay PN beacon signalwith a local reference signal (e.g., local oscillator or PLL). Therecovered timing and phase differences are then removed from the returndownlink signal (e.g., by receive timing and phase adjuster 4024), henceremoving feeder link impairments from the communications signal (e.g.,return downlink signals 527). After feeder link impairment removal, thereturn link signals from a beam will have a common frequency error atall ANs and thus be suitable for beamforming. The common frequency errormay include, but is not limited to, contributions from the user terminalfrequency error, user terminal uplink Doppler, end-to-end relayfrequency translation frequency error and relay PN beacon frequencyerror.

In the forward direction, the access node beacon signal from each AN maybe used to help remove feeder uplink impairments. The feeder uplinkimpairments will be imposed upon the forward link communications data(e.g., the access node-specific signal) as well as the access nodebeacon signal. Coherent demodulation of the access node beacon signalmay be used to recover the timing and phase differences of the accessnode beacon signal (e.g., relative to the relay beacon signal). Therecovered timing and phase differences are then removed from thetransmitted access node beacon signal such that the access node beaconsignal arrives in phase with the relay beacon signal.

In some cases, the forward feeder link removal process is a phase lockedloop (PLL) with the path delay from the AN to the end-to-end relay andback within the loop structure. In some cases, the round-trip delay fromthe AN to the end-to-end relay and back to the AN can be significant.For example, a geosynchronous satellite functioning as an end-to-endrelay will generate round-trip delay of approximately 250 milliseconds(ms). To keep this loop stable in the presence of the large delay, avery low loop bandwidth can be used. For a 250 ms delay, the PLL closedloop bandwidth may typically be less than one Hz. In such cases,high-stability oscillators may be used on both the satellite and the ANto maintain reliable phase lock, as indicated by block 2437 in FIG. 39(see below).

In some cases, the access node beacon signal is a burst signal that isonly transmitted during calibration intervals. During the calibrationinterval, communications data is not transmitted to eliminate thisinterference to the access node beacon signal. Since no communicationsdata is transmitted during the calibration interval, the transmittedpower of the access node beacon signal can be large, as compared to whatwould be required if it were broadcast during communication data. Thisis because there is no concern of causing interference with thecommunications data (the communications data is not present at thistime). This technique enables a strong signal-to-noise ratio (SNR) forthe access node beacon signal when it is transmitted during thecalibration interval. The frequency of occurrence of the calibrationintervals is the reciprocal of the elapsed time between calibrationintervals. Since each calibration interval provides a sample of thephase to the PLL, this calibration frequency is the sample rate of thisdiscrete time PLL. In some cases, the sample rate is high enough tosupport the closed loop bandwidth of the PLL with an insignificantamount of aliasing. The product of the calibration frequency (loopsample rate) and the calibration interval represents the fraction oftime the end-to-end relay cannot be used for communications data withoutadditional interference from the channel sounding probe signal. In somecases, values of less than 0.1 are used and in some cases, values ofless than 0.01 are used.

FIG. 39 is a block diagram of an example AN transceiver 2409. The input2408 to the AN transceiver 2409 receives end-to-end return link signalsreceived by the AN 515 (e.g., for one of a plurality of frequencysub-bands). The input 2408 is coupled to the input 2501 of a downconverter (D/C) 2503. The output of the D/C 2503 is coupled to an analogto digital converter (A/D) 2509. The output of the A/D 2509 is coupledto an Rx time adjuster 2515 and/or Rx phase adjuster 2517. Rx timeadjuster 2515 and Rx phase adjuster 2517 may illustrate aspects of thereceive timing and phase adjuster 4024 of FIG. 38 . The D/C 2503 is aquadrature down converter. Accordingly, the D/C 2503 outputs an in-phaseand quadrature output to the A/D 2509. The received signals may includecommunications signals (e.g., a composite of return uplink signalstransmitted by user terminals), access node beacon signals (e.g.,transmitted from the same AN and/or other ANs) and a relay beaconsignal. The digital samples are coupled to a relay beacon signaldemodulator 2511. The relay beacon signal demodulator 2511 demodulatesthe relay beacon signal. In addition, the relay beacon signaldemodulator 2511 generates a time control signal 2513 and a phasecontrol signal 2514 to remove feeder link impairments based on thereceived relay beacon signal. Such impairments include Doppler, ANfrequency error, scintillation effects, path length changes, etc. Byperforming coherent demodulation of the relay beacon signal, a phaselocked loop (PLL) may be used to correct for most or all of theseerrors. By correcting for the errors in the relay beacon signal,corresponding errors in the communication signals and access node beaconsignals on the feeder link are corrected as well (e.g., since sucherrors are common to the relay beacon signal, the access node beaconsignals and the communications signals). After feeder link impairmentremoval, the end-to-end return link communication signal from a userterminal 517 nominally have the same frequency error at each of the MANs 515. That common error includes the user terminal frequency error,the user link Doppler, the end-to-end relay frequency translation error,and the relay beacon signal frequency error.

The digital samples, with feeder link impairments removed, are coupledto a multiplexer 2518, which may be an example of the multiplexer 4004of FIG. 38 . The multiplexer 2518 associates (e.g., time stamps) thesamples with the relay timing information 2520 from the relay beaconsignal demodulator 2511. The output of the multiplexer 2518 is coupledto the output port 2410 of the AN transceiver 2409. The output port 2410is coupled to the multiplexer 2413 and through the interface 2415 (seeFIG. 40 ) to the CPS 505. The CPS 505 can then use the time stampsassociated with the received digital samples to align the digitalsamples received from each of the ANs 515. Additionally oralternatively, feeder link impairment removal may be performed at theCPS 505. For example, digital samples of the end-to-end return linksignals with the embedded relay beacon signal may be sent from the AN515 to the CPS 505, and the CPS 505 may use the synchronization timinginformation (e.g., embedded relay beacon signal) in each of thecomposite return signals to determine respective adjustments for therespective composite return signals to compensate for downlink channelimpairment.

An access node beacon signal 2530 may be generated locally by an accessnode beacon signal generator 2529. An access node beacon signaldemodulator 2519 demodulates the access node beacon signal received bythe AN 515 (e.g., after being relayed by the end-to-end relay andreceived at input 2408). The relay beacon signal demodulator 2511provides a received relay timing and phase information signal 2521 to acontroller 2523. The controller 2523 also receives a relayed transmittiming and phase information signal 2525 from the access node beaconsignal demodulator 2519. The controller 2523 compares the received relaytiming and phase information with the relayed transmit timing and phaseinformation and generates a coarse time adjust signal 2527. The coarsetime adjust signal 2527 is coupled to the access node beacon signalgenerator 2529. The access node beacon signal generator 2529 generatesthe access node beacon signal 2530 with embedded transmit timinginformation to be transmitted from the AN 515 to the end-to-end relay503. As noted in the discussion above, the difference between the relaytiming and phase information (embedded in the relay beacon signal) andthe transmit time and phase information (embedded in the access nodebeacon signal) is used to adjust the transmit timing and phaseinformation to synchronize the relayed transmit timing and phaseinformation with the received relay timing and phase information. Coarsetime is adjusted by the signal 2527 to the access node beacon signalgenerator 2529 and fine time is adjusted by the signal 2540 to the Txtime adjuster 2539. With the relayed transmit timing and phaseinformation 2525 from the access node beacon signal demodulator 2519synchronized with the received relay timing and phase information 2521,the access node beacon signal generator 2529 generates timestamps 2531that assist in the synchronization of the access node beacon signal 2530and the access node-specific forward signal from the CPS 505 that istransmitted. That is, data samples from the CPS 505 are received oninput port 2423 together with timestamps 2535 that indicate when theassociated data samples is desired to arrive at the end-to-end relay503. A buffer, time align and sum module 2537 buffers the data samplescoupled from the CPS 505 and sums them with the samples from the accessnode beacon signal generator 2529 based on the timestamps 2535, 2531. PNsamples and communication data samples with identical times, asindicated by the time stamps, are summed together. In this example, themultiple beam signals (x_(k)(n)*b_(k)) are summed together in the CPS505 and the access node-specific forward signal comprising a compositeof the multiple beam signals is sent to the AN by the CPS 505.

When aligned properly by the ANs, the data samples arrive at theend-to-end relay 503 at the desired time (e.g., at the same time thatthe same data samples from other ANs arrive). A transmit time adjuster2539 performs fine time adjustments based on a fine time controlleroutput signal 2540 from the time controller module 2523. A transmitphase adjuster 2541 performs phase adjustments to the signal in responseto a phase control signal 2542 generated by the access node beaconsignal demodulator 2519. Transmit time adjuster 2539 and transmit phaseadjuster 2541 may illustrate, for example, aspects of the transmittiming and phase compensator 4020 of FIG. 38 .

The output of the transmit phase adjuster 2541 is coupled to the inputof a digital to analog converter (D/A) 2543. The quadrature analogoutput from the D/A 2543 is coupled to an up-converter (U/C) 2545 to betransmitted by the HPA 2433 (see FIG. 40 ) to the end-to-end relay 503.An amplitude control signal 2547 provided by the access node beaconsignal demodulator 2519 provides amplitude feedback to the U/C 2545 tocompensate for items such as uplink rain fades.

In some cases, the PN code used by each AN for the access node beaconsignal 2530 is different from that used by every other AN. In somecases, the PN codes in the access node beacon signals are each differentfrom the relay PN code used in the relay beacon signal. Accordingly,each AN 515 may be able to distinguish its own access node beacon signalfrom those of the other ANs 515. ANs 515 may distinguish their ownaccess node beacon signals from the relay beacon signal.

As was previously described, the end-to-end gain from any point in thecoverage area to any other point in the area is a multipath channel withL different paths that can result in very deep fades for some point topoint channels. The transmit diversity (forward link) and receivediversity (return link) are very effective in mitigating the deep fadesand enable the communications system to work. However for the accessnode beacon signals, the transmit and receive diversity is not present.As a result, the point-to-point link of a loopback signal, which is thetransmission of the signal from an AN back to the same AN, canexperience end-to-end gains that are much lower than the average. Valuesof 20 dB below the average can occur with a large number ofreceive/transmit signal paths (L). These few low end-to-end gains resultin lower SNR for those ANs and can make link closure a challenge.Accordingly, in some cases, higher gain antennas are used at the ANs.Alternatively, referring to the example transponder of FIG. 16 , a phaseadjuster 418 may be included in each of the receive/transmit signalpaths. The phase adjuster 418 may be individually adjusted by the phaseshift controller 427 (for example, under control of a telemetry,tracking, and command (TT&C) link from an Earth-based control center).Adjusting the relative phases may be effective in increasing theend-to-end gains of the low-gain loopback paths. For example, anobjective may be to choose phase shift settings to increase the value ofthe worst case loopback gain (gain from an AN back to itself). Note thatthe selection of phases generally does not change the distribution ofthe gains when evaluated for all points in the coverage area to allother points in the coverage area, but it can increase the gains of thelow gain loopback paths.

To elaborate, consider the set of gains from each of M ANs 515 to all ofthe other ANs 515. There are M² gains, of which, only M of them areloopback paths. Consider two gain distributions, the first is the totaldistribution of all paths (M²) which can be estimated by compiling ahistogram of all M² paths. For ANs distributed evenly over the entirecoverage area, this distribution may be representative of thedistribution of the end-to-end gain from any point to any other point inthe coverage area. The second distribution is the loopback gaindistribution (loopback distribution) which can be estimated by compilinga histogram of just the M loopback paths. In many cases, customselection of the receive/transmit signal path phase settings (andoptionally gain settings) does not provide a significant change to thetotal distribution. This is especially the case with random orinterleaved mappings of transmit to receive elements. However, in mostcases, the loopback distribution can be improved with custom selection(as opposed to random values) of the phase (and optionally gain)settings. This is because the set of loopback gains consist of M paths(as opposed to M² total paths) and the number of degrees of freedom inthe phase and gain adjustments is L. Often times L is on the same orderas M which enables significant improvement in low loopback gains withcustom phase selection. Another way of looking at this is that thecustom phase selection is not necessarily eliminating low end-to-endgains, but rather moving them from the set of loopback gains (M membersin the set) to the set of non-loopback gains (M²-M members). Fornon-trivial values of M, the larger set is often much larger than theformer.

An AN 515 may process one or more frequency sub-bands. FIG. 40 is ablock diagram of an example AN 515 in which multiple frequency sub-bandsare processed separately. On the end-to-end return link 523 (see FIG. 5), the AN 515 receives the return downlink signals 527 from theend-to-end relay 503 through an LNA 2401. The amplified signals arecoupled from the LNA 2401 to a power divider 2403. The power divider2403 splits the signal into multiple output signals. Each signal isoutput on one of the output ports 2405, 2407 of the power divider 2403.One of the output ports 2407 may be provided as a test port. The otherports 2405 are coupled to an input 2408 of a corresponding one ofmultiple AN transceivers 2409 (only one shown). The AN transceivers 2409process the signals received on corresponding sub-bands. The ANtransceiver 2409 performs several functions, discussed in detail above.The outputs 2410 of the AN transceivers 2409 are coupled to input ports2411 of a sub-band multiplexer 2413. The outputs are combined in thesub-band multiplexer 2413 and output to a distribution network interface2415. The interface 2415 provides an interface for data from/to AN 515to/from the CPS 505 over the distribution network (see FIG. 5 ).Processing frequency sub-bands may be advantageous in reducingperformance requirements on the RF components used to implement theend-to-end relay and AN. For example, by splitting up 3.5 GHz ofbandwidth (e.g., as may be used in a Ka-band system) into sevensub-bands, each sub-band is only 500 MHz wide. That is, each of theaccess node-specific forward signals may include multiple sub-signalsassociated with the different sub-bands (e.g., associated with differentsubsets of the forward user beam coverage areas), and the ANtransceivers 2409 may upconvert the sub-signals to different carrierfrequencies. This bandwidth splitting may allow for lower tolerancecomponents to be used since amplitude and phase variations betweendifferent sub-bands may be compensated by separate beamforming weights,calibration, etc. for the different sub-bands. Of course, other systemsmay use a different number of sub-bands and/or test ports. Some casesmay use a single sub-band and may not include all the components shownhere (e.g., omitting power divider 2403 and mux 2413).

On the end-to-end forward link 501, data is received from the CPS 505 bythe interface 2415. The received data is coupled to an input 2417 of asub-band de-multiplexer 2419. The sub-band de-multiplexer 2419 splitsthe data into multiple data signals. The data signals are coupled fromoutput ports 2421 of the sub-band de-multiplexer 2419 to input ports2423 of the AN transceivers 2409. Output ports 2425 of the ANtransceivers 2409 are coupled to input ports 2427 of the summer module2429. The summer module 2429 sums the signals output from the seven ANtransceivers 2409. An output port 2431 of the summer module 2429 couplesthe output of the summer module 2429 to the input port 2433 of a highpower amplifier (HPA) 2435. The output of the HPA 2435 is coupled to anantenna (not shown) that transmits the signals output to the end-to-endrelay 503. In some cases, an ultra-stable oscillator 2437 is coupled tothe AN transceivers 2409 to provide a stable reference frequency source.

Beam Weight Computation

Returning to FIG. 8 which is an example description of signals on thereturn link, a mathematical model of the end-to-end return link may beused to describe the link as:

$\begin{matrix}\begin{matrix}{y = {{Bret}\left\lfloor {{{Ct}{E\left( {{{Ar}x} + n_{ul}} \right)}} + n_{dl}} \right\rfloor}} \\{= {{Bret}\left\lbrack {{{Hret}x} + {{Ct}{En}_{ul}} + n_{dl}} \right\rbrack}}\end{matrix} & {{EQ}.1}\end{matrix}$

where,

-   -   x is the K×1 column vector of the transmitted signal. In some        cases, the magnitude squared of every element in x is defined to        be unity (equal transmit power). In some cases, this may not        always be the case.    -   y is the K×1 column vector of the received signal after        beamforming.    -   Ar is the L×K return uplink radiation matrix. The element a_(lk)        contains the gain and phase of the path from a reference        location located in beam K to the l^(th) (the letter “el”)        receive antenna element 406 in the Rx array. In some cases, the        values of the return uplink radiation matrix are stored in the        channel data store 941 (see FIG. 30 ).    -   E is the L×L payload matrix. The element e_(ij) defines the gain        and phase of the signal from the j^(th) antenna element 406 in        the receive array to an i^(th) antenna element 409 in the        transmit array. In some cases, aside from incidental crosstalk        between the paths (resulting from the finite isolation of the        electronics), the E matrix is a diagonal matrix. The matrix E        can be normalized such that the sum of the magnitude squared of        all elements in the matrix is L. In some cases, the values of        the payload matrix are stored in the channel data store 941 (see        FIG. 29 ).    -   Ct is the M×L return downlink radiation matrix. The element        c_(ml) contains the gain and phase of the path from l^(th) (the        letter “el”) antenna element in the Tx array to an m^(th) AN 515        from among the M ANs 515. In some cases, the values of the        return downlink radiation matrix are stored in the channel data        store 941 (see FIG. 29 ).    -   Hret is the M×K return channel matrix, which is equal to the        product Ct×E×Ar.    -   n_(ul) is an L×1 noise vector of complex Gaussian noise. The        covariance of the uplink noise is E|n_(ul)n_(ul) ^(H)|=2σ_(ul)        ²I_(L)·I_(L) is the L×L identity matrix.    -   σ² is noise variance. σ_(ul) ² is experienced on the uplink,        while σ_(dl) ² is experienced on the downlink.    -   n_(ld) is an M×1 noise vector of complex Gaussian noise. The        covariance of the downlink noise is E|n_(di)n_(dl) ^(H)|=2σ_(dl)        ²I_(M)·I_(M) is the M×M identity matrix.    -   Bret is the K×M matrix of end-to-end return link beam weights.

Examples are generally described above (e.g., with reference to FIGS.6-11 ) in a manner that assumes certain similarities between forward andreturn end-to-end multipath channels. For example, the forward andreturn channel matrices are described above with reference generally toM, K, E, and other models. However, such descriptions are intended onlyto simplify the description for added clarity and are not intended tolimit examples only to cases with identical configurations in theforward and return directions. For example, in some cases, the sametransponders are used for both forward and return traffic, and thepayload matrix E can be the same for both forward and return end-to-endbeamforming (and corresponding beam weight computations), accordingly.In other cases, different transponders are used for forward and returntraffic, and a different forward payload matrix (Efwd) and a returnpayload matrix (Eret) can be used to model the corresponding end-to-endmultipath channels and to compute corresponding beam weights. Similarly,in some cases, the same M ANs 515 and K user terminals 517 areconsidered part of both the forward and return end-to-end multipathchannels. In other cases, M and K can refer to different subsets of ANs515 and/or user terminals 517, and/or different numbers of ANs 515and/or user terminals 517, in the forward and return directions.

Beam weights may be computed in many ways to satisfy systemrequirements. In some cases, they are computed after deployment of theend-to-end relay. In some cases, the payload matrix E is measured beforedeployment. In some cases, beam weights are computed with the objectiveto increase the signal to interference plus noise (SINR) of each beamand can be computed as follows:

Bret=(R ⁻¹ H)^(H)

R=2σ_(dl) ² I _(M)+2σ_(ul) ² C _(t) EE ^(H) C _(t) ^(H) +HH ^(H)  EQ.2,3

where R is the covariance of the received signal and (*)^(H) is theconjugate transpose (Hermitian) operator.

The A, m element of the K×M return beam weight matrix Bret provides theweights to form the beam to the m^(th) AN 515 from a user terminal inthe k^(th) user beam. Accordingly, in some cases, each of the returnbeam weights used to form return user beams are computed by estimatingend-to-end return gains (i.e., elements of the channel matrix Hret) foreach of the end-to-end multipath channels (e.g., each of the end-to-endreturn multipath channels).

EQ. 2 holds true where R is the covariance of the received signal asprovided in EQ. 3. Therefore, when all of the matrices of EQ. 1, 2 and 3are known, the beam weights used to form end-to-end beams may bedirectly determined.

This set of beam weights reduces the mean squared error between x and y.It also increases the end-to-end signal to noise plus interference ratio(SINR) for each of the K end-to-end return link signals 525 (originatingfrom each of the K beams).

The first term 2σ_(dl) ²I_(M) in EQ. 3 is the covariance of the downlinknoise (which is uncorrelated). The second term 2σ_(dl) ²C_(t)EE^(H)C_(t)^(H) in EQ. 3 is the covariance of the uplink noise (which is correlatedat the ANs). The third term HH^(H) in EQ. 3 is the covariance of thesignal. Setting the variance of the uplink noise to zero and ignoringthe last term (HH^(H)) results a set of weights that increases thesignal to downlink noise ratio by phase-aligning the received signals oneach of the M ANs 515. Setting the downlink noise variance to zero andignoring the 3^(rd) term results in a set of weights that increases theuplink SINR. Setting both the uplink and downlink noise variances tozero results in a de-correlating receiver that increases the carrier tointerference (C/I) ratio.

In some cases, the beam weights are normalized to make the sum of themagnitude squared of any row of Bret sum to unity.

In some cases, the solution to EQ. 2 is determined by a priori knowledgeof the matrices Ar, Ct, and E as well as the variances of the noisevectors n_(ul) and n_(dl). Knowledge of the element values of thematrices can be obtained during measurements made during themanufacturing and testing of relevant components of the end-to-endrelay. This may work well for systems where one does not expect thevalues in the matrices to change significantly during system operation.However, for some systems, especially ones operating in higher frequencybands, such expectations may not be present. In such cases, the matricesAr, Ct, and E may be estimated subsequent to the deployment of a craft(such as a satellite) on which the end-to-end relay is disposed.

In some cases where a priori information is not used to set the weights,the solution to EQ. 2 may be determined by estimating the values of Rand H. In some cases, designated user terminals 517 in the center ofeach user beam coverage area 519 transmit known signals x duringcalibration periods. The vector received at an AN 515 is:

u=Hx+CtEn _(ul) +n _(dl)  EQ. 4

In an example, the CPS 505 estimates the values of R and H based on thefollowing relationships:

{circumflex over (R)}=Σuu ^(H)  EQ. 5

Ĥ=[{circumflex over (p)} ₁ ,{circumflex over (p)} ₂ , . . . {circumflexover (p)} _(K)]  EQ. 6

{circumflex over (p)} _(K) =Σu{tilde over (x)} _(k)*  EQ. 7

{circumflex over (R)} is an estimate of the covariance matrix R, Ĥ is anestimate of channel matrix H and {circumflex over (p)}_(k) is anestimate of the correlation vector, {tilde over (x)}_(k)* is theconjugate of the k^(th) component of the transmitted vector with thefrequency error introduced by the uplink transmission. In some cases, noreturn communication data is transmitted during the calibration period.That is, only calibration signals that are known to the ANs aretransmitted on the end-to-end return link during the calibration periodin order to allow the value of {circumflex over (p)}_(k) to bedetermined from the received vector u using the equation above. This, inturn allows the value of Ĥ to be determined. Both the covariance matrixestimate {circumflex over (R)} and the channel matrix estimate Ĥ aredetermined based on the signals received during the calibration period.

In some cases, the CPS 505 can estimate the covariance matrix{circumflex over (R)} while communication data is present (e.g., evenwhen x is unknown). This may be seen from the fact that {circumflex over(R)} is determined based only on the received signal u. Nonetheless, thevalue of Ĥ is estimated based on signals received during a calibrationperiod during which only calibration signals are transmitted on thereturn link.

In some cases, estimates of both the channel matrix Ĥ and the covariancematrix {circumflex over (R)} are made while communication data is beingtransmitted on the return link. In this case, the covariance matrix{circumflex over (R)} is estimated as noted above. However, the value ofx is determined by demodulating the received signal. Once the value of xis known, the channel matrix may be estimated as noted above in EQ. 6and EQ. 7.

The signal and interference components of the signal after beamformingare contained in the vector Bret H x. The signal and interference powersfor each of the beams are contained in the K×K matrix Bret H. The powerin the k^(th) diagonal element of Bret H is the desired signal powerfrom beam k. The sum of the magnitude squared of all elements in row kexcept the diagonal element is the interference power in beam k. Hencethe C/I for beam k is:

$\begin{matrix}{\left( \frac{C}{I} \right)_{k} = \frac{{❘s_{kk}❘}^{2}}{\sum\limits_{j \neq k}{❘s_{kj}❘}^{2}}} & {{EQ}.8}\end{matrix}$

where s_(kj) are the elements of Bret H. The uplink noise is containedin the vector Bret Ct En_(ul), which has a K×K covariance matrix of2σ_(ul) ²Bret Ct E E^(H) Ct^(H) Bret^(H). The k^(th) diagonal element ofthe covariance matrix contains the uplink noise power in beam k. Theuplink signal to noise ratio for beam k is then computed as:

$\begin{matrix}{\left( \frac{S}{N_{ul}} \right)_{k} = \frac{\left| s_{kk} \right|^{2}}{t_{kk}}} & {{EQ}.9}\end{matrix}$

where ta is the k^(th) diagonal element of the uplink covariance matrix.The downlink noise is contained in the vector Bret n_(dl), which has acovariance of 2σ_(dl) ²I_(K) by virtue of the normalized beam weights.Hence the downlink signal to noise ratio is:

$\begin{matrix}{\left( \frac{S}{N_{dl}} \right)_{k} = \frac{{❘s_{kk}❘}^{2}}{2{\sigma}_{dl}^{2}}} & {{EQ}.10}\end{matrix}$

The end-to-end SINR is the combination of EQ. 8-10:

$\begin{matrix}{{SINR_{k}} = \left\lbrack {\left( \frac{C}{I} \right)_{k}^{- 1} + \left( \frac{S}{N_{ul}} \right)_{k}^{- 1} + \left( \frac{S}{N_{dl}} \right)_{k}^{- 1}} \right\rbrack^{- 1}} & {{EQ}.11}\end{matrix}$

The above equations describe how to calculate the end-to-end SINR giventhe payload matrix E. The payload matrix may be constructed byintelligent choice of the gain and phases of each of the elements of E.The gain and phase of the diagonal elements of E that optimize someutility metric (which is generally a function of the K beam SINR's ascomputed above) may be selected and implemented by setting the phaseshifter 418 in each of the L transponders 411. Candidate utilityfunctions include, but are not limited to, sum of SINR_(k) (total SINR),sum of Log(1+SINR_(k)) (proportional to total throughput) or total powerin the channel matrix, H. In some cases, the improvement in the utilityfunction by customizing the gains and phases is very small andinsignificant. This is sometimes the case when random or interleavedmappings of antenna elements are used. In some cases, the utilityfunction can be improved by a non-trivial amount by custom selection ofthe receive/transmit signal gain and phase.

Returning to FIG. 9 , a mathematical model of the end-to-end forwardlink 501 may be used to describe the link 501 as:

$\begin{matrix}\begin{matrix}{y = {{{At}{E\left\lbrack {{{Cr}{Bfwd}x} + n_{ul}} \right\rbrack}} + n_{dl}}} \\{= {{{Hfwd}{Bfwd}x} + {A{En}_{ul}} + n_{dl}}}\end{matrix} & {{EQ}.12}\end{matrix}$

where,

-   -   x is the K×1 column vector of the transmitted signal. The        magnitude squared of every element in x is defined to be unity        (equal signal power). In some cases, unequal transmit power may        be achieved by selection of the forward beam weights.    -   y is the K×1 column vector of the received signal.    -   Cr is the L×M forward uplink radiation matrix. The element        c_(lm) contains the gain and phase of the path 2002 from m^(th)        AN 515 to the l^(th) (letter “el”) receive antenna element 406        of the Rx array of antenna on the end-to-end relay 503. In some        cases, the values of the forward uplink radiation matrix are        stored in the channel data store 921 (see FIG. 29 ).    -   E is the L×L payload matrix. The element e_(ij) defines the gain        and phase of the signal from j^(th) receive array antenna        element to the i^(th) antenna element of the transmit array.        Aside from incidental crosstalk between the paths (resulting        from the finite isolation of the electronics), the E matrix is a        diagonal matrix. In some cases, the matrix E is normalized such        that the sum of the magnitude squared of all elements in the        matrix is L. In some cases, the values of the payload matrix are        stored in the channel data store 921 (see FIG. 29 ).    -   At is the K×L forward downlink radiation matrix. The element au        contains the gain and phase of the path from antenna element L        (letter “el”) in the Tx array of the end-to-end relay 503 to a        reference location in user beam k. In some cases, the values of        the forward downlink radiation matrix are stored in the channel        data store 921 (see FIG. 29 ).    -   Hfwd is the K×M forward channel matrix, which is equal to the        product A_(t)EC_(r).    -   n_(ul) is an L×1 noise vector of complex Gaussian noise. The        covariance of the uplink noise is:

E[n_(ul)n_(ul)^(H)] = 2σ_(ul)²I_(L),

-   -   where I_(L) is the L×L identity matrix.    -   n_(dl) is an K×1 noise vector of complex Gaussian noise. The        covariance of the downlink noise is:

E[n_(dl)n_(dl)^(H)] = 2σ_(dl)²I_(K),

where I_(K) is the K×K identity matrix.

-   -   Bfwd is the M×K beam weight matrix of end-to-end forward link        beam weights.

The beam weights for user beam k are the elements in column k of Bfwd.Unlike the return link, the C/I for beam k is not determined by the beamweights for beam k. The beam weights for beam k determine the uplinksignal to noise ratio (SNR) and the downlink SNR, as well as the carrier(C) power in the C/I. However, the interference power in beam k isdetermined by the beam weights for all of the other beams, except forbeam k. In some cases, the beam weight for beam k is selected toincrease the SNR. Such beam weights also increase the C/I for beam k,since C is increased. However, interference may be generated to theother beams. Thus, unlike in the case of the return link, optimal beamweights are not computed on a beam-by-beam basis (independent of theother beams).

In some cases, beam weights (including the radiation and payloadmatrices used to compute them) are determined after deployment of theend-to-end relay. In some cases, the payload matrix E is measured beforedeployment. In some cases, one can compute a set of beam weights byusing the interference created in the other beams by beam k and countingit as the interference in beam k. Although this approach may not computeoptimum beam weights, it may be used to simplify weight computation.This allows a set of weights to be determined for each beam independentof all other beams. The resulting forward beam weights are then computedsimilar to the return beam weights:

Bfwd=H ^(H) R ⁻¹,  EQ. 13

where,

R=2σ_(dl) ² I _(K)+2σ_(ul) ²AtEE^(H) At _(t) ^(H) +HH ^(H)  EQ. 14

The first term 2σ_(dl) ²I_(K) in EQ. 14 is the covariance of thedownlink noise (uncorrelated). The second term 2σ_(ul) ²At EE^(H) At^(H)is the covariance of the uplink noise (which is correlated at the ANs).The third term HH^(H) is the covariance of the signal. Setting thevariance of the uplink noise to zero and ignoring the last term (HH^(H))results in a set of weights that increases the signal to downlink noiseratio by phase aligning the received signals at the M ANs 515. Settingthe downlink noise variance to zero and ignoring the 3^(rd) term resultsin a set of weights that increases the uplink SNR. Setting both theuplink and downlink noise variances to zero results in a de-correlatingreceiver that increases the C/I ratio. For the forward link, thedownlink noise and interference generally dominate. Therefore, theseterms are generally useful in the beam weight computation. In somecases, the second term in EQ. 14 (the uplink noise) is insignificantcompared to the first term (the downlink noise). In such cases, thesecond term can be ignored in co-variance calculations, furthersimplifying the calculation while still yielding a set of beam weightsthat increases the end-to-end SINR.

As with the return link, the beam weights may be normalized. Fortransmitter beam weights with equal power allocated to all K forwardlink signals, each column of Bfwd may be scaled such that the sum of themagnitude squared of the elements in any column will sum to unity. Equalpower sharing will give each of the signals the same fraction of totalAN power (total power from all ANs allocated to signal x_(k)). In somecases, for forward links, an unequal power sharing between forward linksignals is implemented. Accordingly, in some cases, some beam signalsget more than an equal share of total AN power. This may be used toequalize the SINR in all beams or give more important beams largerSINR's than lesser important beams. To create the beam weights forunequal power sharing, the M×K equal power beam weight matrix, Bfwd, ispost multiplied by a K×K diagonal matrix, P, thus the new Bfwd=Bfwd P.Let

P=diag(√{square root over (p _(k))}),

then the squared valued of the k^(th) diagonal element represents thepower allocated to user signal x_(k). The power sharing matrix P isnormalized such that the sum or the square of the diagonal elementsequals K (the non-diagonal elements are zero).

In some cases, the solution to EQ. 13 is determined by a prioriknowledge of the matrices At, C_(r), and E, as well as the variances ofthe noise vectors n_(ul) and n_(dl). In some cases, knowledge of thematrices can be obtained during measurements made during themanufacturing and testing of relevant components of the end-to-endrelay. This can work well for systems where one does not expect thevalues in the matrices to change significantly from what was measuredduring system operation. However, for some systems, especially onesoperating in higher frequency bands, this may not be the case.

In some cases where a priori information is not used to set the weights,the values of R and H for the forward link can be estimated to determinethe solution to EQ. 13. In some cases, ANs transmit a channel soundingprobe during calibration periods. The channel sounding probes can bemany different types of signals. In one case, different, orthogonal andknown PN sequences are transmitted by each AN. The channel soundingprobes may be pre-corrected in time, frequency, and/or phase to removethe feeder link impairments (as discussed further below). Allcommunication data may be turned off during the calibration interval toreduce the interference to the channel sounding probes. In some cases,the channel sounding probes can be the same signals as those used forfeeder link impairment removal.

During the calibration interval, a terminal in the center of each beammay be designated to receive and process the channel sounding probes.The K×l vector, u, of received signals during the calibration period isu=H x+At E n_(ul)+n_(dl) where x is the M×l vector of transmittedchannel sounding probes. In some cases, each designated terminal firstremoves the incidental frequency error (resulting from Doppler shift andterminal oscillator error), and then correlates the resulting signalwith each of the M known, orthogonal PN sequences. The results of thesecorrelations are M complex numbers (amplitude and phase) for eachterminal and these results are transmitted back to the CPS via thereturn link. The M complex numbers calculated by the terminal in thecenter of the k^(th) beam can be used to form the k^(th) row of theestimate of the channel matrix, Ĥ. By using the measurements from all ofK designated terminals, an estimate of the entire channel matrix isobtained. In many cases, it is useful to combine the measurement frommultiple calibration intervals to improve the estimate of the channelmatrix. Once the estimate of the channel matrix is determined, anestimate of the covariance matrix, {circumflex over (R)}, can bedetermined from EQ. 14 using a value of 0 for the second term. This maybe a very accurate estimate of the covariance matrix if the uplink noise(the second term in EQ. 14) is negligible relative to the downlink noise(the first term in EQ. 14). The forward link beam weights may then becomputed by using the estimates of the channel matrix and covariancematrix in EQ. 13. Accordingly, in some cases, the computation of beamweights comprises estimating end-to-end forward gains (i.e., the valuesof the elements of the channel matrix Hfwd) for each of the end-to-endforward multipath channels between an AN 515 and a reference location ina user beam coverage area. In other cases, computation of beam weightscomprises estimating end-to-end forward gains for K×M end-to-end forwardmultipath channels from M ANs 515 to reference locations located withinK user beam coverage areas.

The signal and interference components of the signal after beamformingare contained in the vector H Bfwd x (product of H, Bfwd, x). The signaland interference powers for each of the beams are contained in the K×Kmatrix H Bfwd. The power in the k^(th) diagonal element of H Bfwd is thedesired signal power intended for beam k. The sum of the magnitudesquared of all elements in row k except the diagonal element is theinterference power in beam k. Hence the C/I for beam k is:

$\begin{matrix}{\left( \frac{C}{I} \right)_{k} = \frac{{❘s_{kk}❘}^{2}}{\sum\limits_{j \neq k}{❘s_{kj}❘}^{2}}} & {{EQ}.15}\end{matrix}$

where s_(kj) are the elements of H B fwd. The uplink noise is containedin the vector A_(t) E n_(ul), which has a K×K covariance matrix of2σ_(ul) ² EE^(H) At_(t) ^(H). The k^(th) diagonal element of thecovariance matrix contains the uplink noise power in beam k. The uplinksignal to noise ratio for beam k is then computed as:

$\begin{matrix}{\left( \frac{S}{N_{ul}} \right)_{k} = \frac{\left| s_{kk} \right|^{2}}{t_{kk}}} & {{EQ}.16}\end{matrix}$

where t_(kk) is the k^(th) diagonal element of the uplink covariancematrix. The downlink noise is contained in the vector n_(dl), which hasa covariance of 2σ_(dl) ²I_(K). Hence the downlink signal to noise ratiois:

$\begin{matrix}{\left( \frac{S}{N_{dl}} \right)_{k} = \frac{{❘{s}_{kk}❘}^{2}}{2{\sigma}_{dl}^{2}}} & {{EQ}.17}\end{matrix}$

The end-to-end SINR is the combination of EQ. 15-EQ. 17:

$\begin{matrix}{{SINR_{k}} = \left\lbrack {\left( \frac{C}{I} \right)_{k}^{- 1} + \left( \frac{S}{N_{ul}} \right)_{k}^{- 1} + \left( \frac{S}{N_{dl}} \right)_{k}^{- 1}} \right\rbrack^{- 1}} & {{EQ}.18}\end{matrix}$

The above equations describe how to calculate the end-to-end SINR giventhe payload matrix E. The payload matrix may be constructed byintelligent choice of the gain and phases of each of the elements of E.The gain and phase of the diagonal elements of E that optimize someutility metric (which is generally a function of the K beam SINR's ascomputed above) may be selected and implemented by setting the phaseshifter 418 in each of the L transponders 411. Candidate utilityfunctions include, but are not limited to, sum of SINR_(k) (total SINR),sum of Log(1+SINR_(k)) (proportional to total throughput) or total powerin the channel matrix, H. In some cases, the improvement in the utilityfunction by customizing the gains and phases is very small andinsignificant. This is sometimes the case when random or interleavedmappings of antenna elements are used. In some cases, the utilityfunction can be improved by a non-trivial amount by custom selection ofthe receive/transmit signal gain and phase.

Distinct Coverage Areas

Some examples described above assume that the end-to-end relay 503 isdesigned to service a single coverage area shared by both the userterminals 517 and the ANs 515. For example, some cases describe asatellite having an antenna subsystem that illuminates a satellitecoverage area, and both the ANs 515 and the user terminals 517 aregeographically distributed throughout the satellite coverage area (e.g.,as in FIG. 27 ). The number of beams that can be formed in the satellitecoverage area, and the sizes (beam coverage areas) of those beams can beaffected by aspects of the antenna subsystem design, such as number andarrangement of antenna elements, reflector size, etc. For example,realizing a very large capacity can involve deploying a large number(e.g., hundreds) of ANs 515 with sufficient spacing between the ANs 515to allow for end-to-end beamforming. For example, as noted above withreference to FIG. 28 , increasing the number of ANs 515 can increasesystem capacity, although with diminishing returns as the numberincreases. When one antenna subsystem supports both the user terminals517 and the ANs 515, achieving such a deployment with sufficient spacingbetween ANs 515 can force a very wide geographical distribution of theANs 515 (e.g., across the entire satellite coverage area, as in FIG. 27). Practically, achieving such a distribution may involve placing ANs515 in undesirable locations, such as in areas with poor access to ahigh-speed network (e.g., a poor fiber infrastructure back to the CPS505), multiple legal jurisdictions, in expensive and/or highly populatedareas, etc. Accordingly, AN 515 placement often involves varioustradeoffs.

Some examples of the end-to-end relay 503 are designed with multipleantenna subsystems, thereby enabling separate servicing of two or moredistinct coverage areas from a single end-to-end relay 503. As describedbelow, the end-to-end relay 503 can include at least a first antennasubsystem that services an AN area 3450, and at least a second antennasubsystem that services a user coverage area 3460. Because the usercoverage area 3460 and AN area 3450 may be serviced by different antennasubsystems, each antenna subsystem can be designed to meet differentdesign parameters, and each coverage area can be at least partiallydistinct (e.g., in geography, in beam size and/or density, in frequencyband, etc.). For example, using such a multi-antenna subsystem approachcan enable user terminals 517 distributed over one or more relativelylarge geographic areas 3460 (e.g., the entire United States) to beserviced by a large number of ANs 515 distributed over one or morerelatively small geographic areas (e.g., a portion of the Eastern UnitedStates). For example, the AN area 3450 can be a fraction (e.g., lessthan one half, less than one quarter, less than one fifth, less than onetenth) of the user coverage area 3460 in physical area.

FIG. 41 is an illustration of an example end-to-end beamforming system3400. The system 3400 is an end-to-end beamforming system that includes:a plurality of geographically distributed ANs 515; an end-to-end relay3403; and a plurality of user terminals 517. The end-to-end relay 3403can be an example of end-to-end relay 503 described herein. The ANs 515are geographically distributed in an AN area 3450, the user terminals517 are geographically distributed in a user coverage area 3460. The ANarea 3450 and the user coverage area 3460 are both within the visibleEarth coverage area of the end-to-end relay 3403, but the AN area 3450is distinct from the user coverage area 3460. In other words, the ANarea 3450 is not coextensive with the user coverage area 3460 but mayoverlap at least partially with the user coverage area 3460. However,the AN area 3450 may have a substantial (non-trivial) area (e.g., morethan one-tenth, one-quarter, one-half, etc. of the AN area 3450) thatdoes not overlap with the user coverage area 3460. For example, in somecases, at least half of the AN area 3450 does not overlap the usercoverage area 3460. In some cases, the AN area 3450 and user coveragearea 3460 may not overlap at all, as discussed with reference to FIG.45C. As described above (e.g., in FIG. 5 ), the ANs 515 can exchangesignals through a distribution network 518 with a CPS 505 within aground segment 502, and the CPS 505 can be connected to a data source.

The end-to-end relay 3403 includes a separate feeder-link antennasubsystem 3410 and user-link antenna subsystem 3420. Each of thefeeder-link antenna subsystem 3410 and the user-link antenna subsystem3420 is capable of supporting end-to-end beamforming. For example, asdescribed below, each antenna subsystem can have its own array(s) ofcooperating antenna elements, its own reflector(s), etc. The feeder-linkantenna subsystem 3410 can include an array 3415 of cooperatingfeeder-link constituent receive elements 3416 and an array 3415 ofcooperating feeder-link constituent transmit elements 3419. Theuser-link antenna subsystem 3420 can include an array 3425 ofcooperating user-link constituent receive elements 3426 and an array3425 of cooperating user-link constituent transmit elements 3429. Theconstituent elements are “cooperating” in the sense that the array ofsuch constituent elements has characteristics making its respectiveantenna subsystem suitable for use in a beamforming system. For example,a given user-link constituent receive element 3426 can receive asuperposed composite of return uplink signals 525 from multiple (e.g.,some or all) user beam coverage areas 519 in a manner that contributesto forming of return user beams. A given user-link constituent transmitelement 3429 can transmit a forward downlink signal 522 in a manner thatsuperposes with corresponding transmissions from other user-linkconstituent transmit elements 3429 to form some or all forward userbeams. A given feeder-link constituent receive element 3416 can receivea superposed composite of forward uplink signals 521 from multiple(e.g., all) ANs 515 in a manner that contributes to forming of forwarduser beams (e.g., by inducing multipath at the end-to-end relay 3403). Agiven feeder-link constituent transmit element 3419 can transmit areturn downlink signal 527 in a manner that superposes withcorresponding transmissions from other feeder-link constituent transmitelements 3419 to contribute to forming of some or all return user beams(e.g., by enabling the ANs 515 to receive composite return signals thatcan be beam-weighted to form the return user beams).

The example end-to-end relay 3403 includes a plurality of forward-linktransponders 3430 and a plurality of return-link transponders 3440. Thetransponders can be any suitable type of bent-pipe signal path betweenthe antenna subsystems. Each forward-link transponder 3430 couples arespective one of the feeder-link constituent receive elements 3416 witha respective one of the user-link constituent transmit elements 3429.Each return-link transponder 3440 couples a respective one of theuser-link constituent receive elements 3426 with a respective one of thefeeder-link constituent transmit elements 3419. Some examples aredescribed as having a one-to-one correspondence between each user-linkconstituent receive element 3426 and a respective feeder-linkconstituent transmit element 3419 (or vice versa), or that eachuser-link constituent receive element 3426 is coupled with “one and onlyone” feeder-link constituent transmit element 3419 (or vice versa), orthe like. In some such cases, one side of each transponder is coupledwith a single receive element, and the other side of the transponder iscoupled with a single transmit element. In other such cases, one or bothsides of a transponder can be selectively coupled (e.g., by a switch,splitter, combiner, or other means, as described below) with one ofmultiple elements. For example, the end-to-end relay 3403 can includeone feeder-link antenna subsystem 3410 and two user-link antennasubsystems 3420; and each transponder can be coupled, on one side, to asingle feeder-link element, and selectively coupled, on the other side,either to a single user-link element of the first user-link antennasubsystem 3420 or to a single user-link element of the second user-linkantenna subsystem 3420. In such selectively coupled cases, each side ofeach transponder can still be considered at any given time (e.g., for aparticular signal-related transaction) as being coupled with “one andonly one” element, or the like.

For forward communications, transmissions from the ANs 515 can bereceived (via feeder uplinks 521) by the feeder-link constituent receiveelements 3416, relayed by the forward-link transponders 3430 to theuser-link constituent transmit elements 3429, and transmitted (via userdownlinks 522) by the user-link constituent transmit elements 3429 touser terminals 517 in the user coverage area 3460. For returncommunications, transmissions from the user terminals 517 can bereceived (via user uplink signals 525) by user-link constituent receiveelements, relayed by the return-link transponders 3440 to thefeeder-link constituent transmit elements 3419, and transmitted by thefeeder-link constituent transmit elements 3419 to ANs 515 in the AN area3450 (via feeder downlink signals 527). The full signal path from an AN515 to a user terminal 517 via the end-to-end relay 3403 is referred toas the end-to-end forward link 501; and the full signal path from a userterminal 517 to an AN 515 via the end-to-end relay 3403 is referred toas the end-to-end return link 523. As described herein, the end-to-endforward link 501 and the end-to-end return link 523 can each includemultiple multipath channels for forward and return communications.

In some cases, each of the plurality of geographically distributed ANs515 has an end-to-end beam-weighted forward uplink signal 521 output.The end-to-end relay 3403 comprises an array 3415 of cooperatingfeeder-link constituent receive elements 3416 in wireless communicationwith the distributed ANs 515, an array 3425 of cooperating user-linkconstituent transmit elements 3429 in wireless communication with theplurality of user terminals 517, and a plurality of forward-linktransponders 3430. The forward-link transponders 3430 may be “bent-pipe”(or non-processing) transponders, so that each transponder outputs asignal that corresponds to the signal it receives with little or noprocessing. For example, each forward-link transponder 3430 can amplifyand/or frequency translate its received signal, but may not perform morecomplex processing (e.g., there is no analog-to-digital conversion,demodulation and/or modulation, no on-board beamforming, etc.). In somecases, each forward-link transponder 3430 accepts an input at a firstfrequency range (e.g., 30 GHz LHCP) and outputs at a second frequencyrange (e.g., 20 GHz RHCP), and each return-link transponder 3440 acceptsan input at the first frequency range (e.g., 30 GHz RHCP) and outputs atthe second frequency range (e.g., 20 GHz LHCP). Any suitable combinationof frequency and/or polarization can be used, and the user-link andfeeder-link can use the same or different frequency ranges. As usedherein, a frequency range refers to a set of frequencies used for signaltransmission/reception and may be a contiguous range or include multiplenon-contiguous ranges (e.g., such that a given frequency range maycontain frequencies from more than one frequency band, a given frequencyband may contain multiple frequency ranges, etc.). Each forward-linktransponder 3430 is coupled between a respective one of the feeder-linkconstituent receive elements 3416 and a respective one of the user-linkconstituent transmit elements 3419 (e.g., with a one-to-onecorrespondence). The forward-link transponders 3430 convertsuperpositions of a plurality of beam-weighted forward uplink signals521 received via the feeder-link constituent receive elements 3416 intoforward downlink signals 522. Transmission of the forward downlinksignals 522 by the user-link constituent transmit elements 3429contributes to forming a forward user beam servicing at least some ofthe plurality of user terminals 517 (e.g., which may be grouped into oneor more user beam coverage areas 519 for transmissions via correspondingbeamformed forward user beams). As described herein, the forward uplinksignals 521 can be end-to-end beam-weighted and synchronized (e.g.,phase-synchronized, and, if desired, time-synchronized) prior totransmission from the ANs 515, which can enable the desiredsuperposition of those signals 521 at the feeder-link constituentreceive elements 3416.

The transmission of the forward uplink signals 521 contributes toforming the forward user beam in the sense that the beamforming isend-to-end, as described herein; the beamforming is a result of multiplesteps, including computing and applying appropriate weights to theforward uplink signals 521 prior to transmission to the relay from theANs 515, inducing multipath reception by the multiple forward-linktransponders 3430 of the end-to-end relay 3403, and transmitting theforward downlink signals 522 from multiple user-link constituenttransmit elements 3429. Still, for the sake of simplicity, somedescriptions can refer to the forward beam as being formed bysuperposition of the transmitted forward downlink signals 522. In somecases, each of the plurality of user terminals 517 is in wirelesscommunication with the array 3425 of cooperating user-link constituenttransmit elements 3429 to receive a composite (e.g., a superposition) ofthe transmitted forward downlink signals 522.

In some cases, the end-to-end relay 3403 further includes an array 3425of user-link constituent receive elements 3426 in wireless communicationwith the user terminals 517, an array 3415 of cooperating feeder-linkconstituent transmit elements 3419 in wireless communication with thedistributed ANs 515, and a plurality of return-link transponders 3440.The return-link transponders 3440 can be similar or identical to theforward-link transponders 3430 (e.g., bent-pipe transponders), exceptthat each is coupled between a respective one of the user-linkconstituent receive elements 3426 and a respective one of thefeeder-link constituent transmit elements 3419. Receipt of return uplinksignals 525 via the array of cooperating user-link constituent receiveelement 3426 allows the formation of return downlink signals 527 in thereturn-link transponders 3440. In some cases, each return downlinksignal 527 is a respective superposition of return uplink signals 525received by a user-link constituent receive element 3426 from multipleuser terminals 517 (e.g., from one or more user beam coverage areas519). In some such cases, each of the plurality of user terminals 517 isin wireless communication with the array of cooperating user-linkconstituent receive elements 3426 to transmit a respective return uplinksignal 525 to multiple of the user-link constituent receive elements3426.

In some cases, the return downlink signals 527 are transmitted by thefeeder-link constituent transmit elements 3419 to the geographicallydistributed ANs 515. As described herein, each AN 515 can receive asuperposed composite of the return downlink signals 527 transmitted fromthe feeder-link constituent transmit elements 3419. The superposedcomposite may be an example of superposition 1706 described withreference to FIG. 6 . The received return downlink signals 527 (whichmay be referred to as composite return signals) can be coupled to areturn beamformer 531, which can combine, synchronize, beam weight, andperform any other suitable processing. For example, the returnbeamformer 531 can weight the received superpositions 1706 of the returndownlink signals 527 (i.e., apply return beam weights to the compositereturn signals) prior to combining the signals. The return beamformer531 can also synchronize the composite return signals 1706 prior tocombining the signals to account at least for respective path delaydifferences between the end-to-end relay 3403 and the ANs 515. In somecases, the synchronizing can be according to a received beacon signal(received by one or more, or all, of the ANs 515).

Because of the end-to-end nature of the beamforming, proper applicationof return beam weights by the return beamformer 531 enables formation ofthe return user beams, even though the return beamformer 531 may becoupled to the feeder-link side of the end-to-end multipath channels,and the user beams may be formed at the user-link side of the end-to-endmultipath channels. Accordingly, the return beamformer 531 can bereferred to as contributing to the forming of the return user beams (anumber of other aspects of the system 3400 also contribute to theend-to-end return beamforming, such as the inducement of multipath bythe return-link transponders 3440 of the end-to-end relay 3403). Still,the return beamformer 531 can be referred to as forming the return userbeams for the sake of simplicity.

In some cases, the end-to-end relay 3403 further includes a feeder-linkantenna subsystem 3410 to illuminate an AN area 3450 within which theANs 515 are distributed. The feeder-link antenna subsystem 3410comprises the array 3415 of cooperating feeder-link constituent receiveelements 3416. In some cases, the end-to-end relay 3403 also includes auser-link antenna subsystem 3420 to illuminate a user coverage area 3460within which the plurality of user terminals 517 is geographicallydistributed (e.g., in a plurality of user beam coverage areas 519). Theuser-link antenna subsystem 3420 comprises the array 3425 of cooperatinguser-link constituent transmit elements 3429. In some cases, theuser-link antenna subsystem 3420 includes a user-link receive array anda user-link transmit array (e.g., separate, half-duplex arrays ofcooperating user-link constituent elements). The user-link receive arrayand the user-link transmit array can be spatially interleaved (e.g., topoint to a same reflector), spatially separated (e.g., to point atreceive and transmit reflectors, respectively), or arranged in any othersuitable manner (e.g., as discussed with reference to FIG. 62 ). Inother cases, the user-link antenna subsystem 3420 includes full-duplexelements (e.g., each user-link constituent transmit element 3429 sharesradiating structure with a respective user-link constituent receiveelement 3426). Similarly, in some cases, the feeder-link antennasubsystem 3410 includes a feeder-link receive array and a feeder-linktransmit array, which may be spatially related in any suitable mannerand may directly radiate, point to a single reflector, point to separatetransmit and receive reflectors, etc. In other cases, the feeder-linkantenna subsystem 3410 includes full-duplex elements. The feeder-linkantenna subsystem 3410 and the user-link antenna subsystem 3420 can havethe same or different aperture sizes. In some cases, the feeder-linkantenna subsystem 3410 and the user-link antenna subsystem 3420 operatein a same frequency range (e.g., a frequency range within the K/Ka band,etc.). In some cases, the feeder-link antenna subsystem 3410 and theuser-link antenna subsystem 3420 operate in different frequency ranges(e.g., feeder-link uses V/W band, the user-link uses K/Ka band, etc.).In some cases, the feeder-link antenna subsystem 3410 and/or theuser-link antenna subsystem 3420 may operate in multiple frequencyranges (e.g., feeder-link uses V/W band and K/Ka-band, as describedbelow with reference to FIG. 64A, 64B, 65A or 65B).

In examples, such as those illustrated by FIG. 41 , the AN area 3450 isdistinct from the user coverage area 3460. The AN area 3450 can be asingle, contiguous coverage area, or multiple disjoint coverage areas.Similarly (and independently of whether the AN area 3450 is single ormultiple), the user coverage area 3460 can be a single, contiguouscoverage area, or multiple disjoint coverage areas. In some cases, theAN area 3450 is a subset of the user coverage area 3460. In some cases,at least half of the user coverage area 3460 does not overlap the ANarea 3450. As described below, in some cases, the feeder-link antennasubsystem 3410 further comprises one or more feeder-link reflectors, andthe user-link antenna subsystem 3420 further comprises one or moreuser-link reflectors. In some cases, the feeder-link reflector issignificantly larger (e.g., at least twice the physical area, at leastfive times, ten times, fifty times, eighty times, etc.) than theuser-link reflector. In some cases, the feeder-link reflector isapproximately the same physical area (e.g., within 5%, 10%, 25%) as theuser-link reflector.

In some cases, the system 3400 operates in the context of ground networkfunctions, as described with reference to FIG. 5 . For example, theend-to-end relay 3403 communicates with ANs 515, which communicate witha CPS 505 via a distribution network 518. In some cases, the CPS 505includes a forward beamformer 529 and/or a return beamformer 531, forexample, as described with reference to FIG. 29 . As described above,the forward beamformer 529 can participate in forming forward end-to-endbeams by applying computed forward beam weights (e.g., supplied by aforward beam weight generator 917) to forward uplink signals 521; andthe return beamformer 531 can participate in forming return end-to-endbeams by applying computed return beam weights (e.g., supplied by areturn beam weight generator 935) to return downlink signals 527. Asdescribed above, the end-to-end forward beam weights and/or the set ofend-to-end return beam weights can be computed according to estimatedend-to-end gains for end-to-end multipath channels, each end-to-endmultipath channel communicatively coupling a respective one of thedistributed ANs 515 with a respective location in the user coverage area3460 (e.g., a user terminal 517 or any suitable reference location) viaa respective plurality of the forward-link bent-pipe transponders 3430and/or via a respective plurality of the return-link bent-pipetransponders 3440. In some cases, though not shown, the end-to-end relay3403 includes a beacon signal transmitter. The beacon signal transmittercan be implemented as described above with reference to the beaconsignal generator and calibration support module 424 of FIG. 15 . In somecases, the generated beacon signal can be used so that the plurality ofdistributed ANs 515 is in time-synchronized wireless communication withthe end-to-end relay 3403 (e.g., with the plurality of feeder-linkconstituent receive elements 3416 according to the beacon signal).

In some cases, the system 3400 includes a system for forming a pluralityof forward user beams using end-to-end beamforming. Such cases includemeans for transmitting a plurality of forward uplink signals 521 from aplurality of geographically distributed locations, wherein the pluralityof forward uplink signals 521 is formed from a weighted combination of aplurality of user beam signals, and wherein each user beam signalcorresponds to one and only one user beam. For example, the plurality ofgeographically distributed locations can include a plurality of ANs 515,and the means for transmitting the plurality of forward uplink signals521 can include some or all of a forward beamformer 529, a distributionnetwork 518, and the geographically distributed ANs 515 (incommunication with the end-to-end relay 3403). Such cases can alsoinclude means for relaying the plurality of forward uplink signals 521to form a plurality of forward downlink signals 522. Each forwarddownlink signal 522 is created by amplifying a unique superposition ofthe plurality of forward uplink signals 521, and the plurality offorward downlink signals 522 superpose to form the plurality of userbeams, wherein each user beam signal is dominant within thecorresponding user beam coverage area 519. For example, the means forrelaying the plurality of forward uplink signals 521 to form theplurality of forward downlink signals 522 can include the end-to-endrelay 3403 (in communication with one or more user terminals 517 in userbeam coverage areas 519) with its collocated plurality of signal paths,which can include forward-link transponders 3430 and return-linktransponders 3440.

Some such cases include first means for receiving a first superpositionof the plurality of forward downlink signals 522 and recovering a firstone of the plurality of user beam signals. Such first means can includea user terminal 517 (e.g., including a user terminal antenna, and amodem or other components for recovering user beam signals from theforward downlink signals). Some such cases also include second means(e.g., including a second user terminal 517) for receiving a secondsuperposition of the plurality of forward downlink signals 522 andrecovering a second one of the plurality of user beam signals. Forexample, the first means for receiving is located within a first userbeam coverage area 519, and the second means for receiving is locatedwithin a second user beam coverage area 519.

FIG. 42 is an illustration of an example model of signal paths forsignals carrying return data on the end-to-end return link 523. Theexample model can operate similarly to the model described withreference to FIGS. 6-8 , except that the end-to-end relay 3403 includesreturn-link signal paths 3502 dedicated for return-link communications.Each return-link signal path 3502 can include a return-link transponder3440 coupled (e.g., selectively coupled) between a user-link constituentreceive element 3426 and a feeder-link constituent transmit element3419. Signals originating with user terminals 517 in K user beamcoverage areas 519 are transmitted (as return uplink signals 525) to theend-to-end relay 3403, received by an array of L user-link constituentreceive elements 3426, communicated through L return-link signal paths3502 (e.g., via L return-link transponders 3440) to L correspondingfeeder-link constituent transmit elements 3419, and transmitted by eachof the L feeder-link constituent transmit elements 3419 to some or allof the M ANs 515 (similar to what is shown in FIG. 7 ). In this way, themultiple return-link signal paths 3502 (e.g., the return-linktransponders 3440) induce multipath in the return-link communications.For example, the output of each return-link signal path 3502 is a returndownlink signal 527 corresponding to a received composite of the returnuplink signals 525 transmitted from multiple of the user beam coverageareas 519, and each return downlink signal 527 is transmitted to some orall of the M ANs 515 (e.g., geographically distributed over an AN area3450). Accordingly, each AN 515 may receive a superposition 1706 of someor all of the return downlink signals 527, which may then becommunicated to a return beamformer 531. As described above, there are L(or up to L) different ways for a signal to get from a user terminal 517located in a user beam coverage area 519 to a particular AN 515. Theend-to-end relay 3403 thereby creates L paths between a user terminal517 and an AN 515, referred to collectively as an end-to-end returnmultipath channel 1908 (e.g., similar to FIG. 8 ).

The end-to-end return multipath channels can be modeled in the samemanner described above. For example, Ar is the L×K return uplinkradiation matrix, Ct is the M×L return downlink radiation matrix, andEret is the L×L return payload matrix for the paths from the user-linkconstituent receive elements 3426 to the feeder-link constituenttransmit elements 3419. As described above, the end-to-end returnmultipath channel from a user terminal 517 in a particular user beamcoverage area 519 to a particular AN 515 is the net effect of the Ldifferent signal paths induced by L unique return-link signal paths 3502through the end-to-end relay 3403. With K user beam coverage areas 519and M ANs 515, there can be M×K induced end-to-end return multipathchannels in the end-to-end return link 523 (via the end-to-end relay3403), and each can be individually modeled to compute a correspondingelement of an M×K return channel matrix Hret (C_(t)×Eret×Ar). As notedabove (e.g., with reference to FIGS. 6-8 ), not all ANs 515, user beamcoverage areas 519, and/or return-link transponders 3440 have toparticipate in the end-to-end return multipath channels. In some cases,the number of user beams K is greater than the number of transponders Lin the signal path of the end-to-end return multipath channel; and/orthe number of ANs 515 M is greater than the number of return-linktransponders 3440 L in the signal path of the end-to-end returnmultipath channel. As described with reference to FIG. 5 , the CPS 505can enable forming of return user beams by applying return beam weightsto the received downlink return signals 527 (the received signals, afterreception by the AN 515 are referred to as composite return signals 907,as explained further below). The return beam weights can be computedbased on the model of the M×K signal paths for each end-to-end returnmultipath channel that couples the user terminals 517 in one user beamcoverage area 519 with one of the plurality of ANs 515.

FIG. 43 is an illustration of an example model of signal paths forsignals carrying forward data on the end-to-end forward link 501. Theexample model can operate similarly to the model described withreference to FIGS. 9-11 , except that the end-to-end relay 3403 includesforward-link signal paths 3602 dedicated for forward-linkcommunications. Each forward-link signal path 3602 can include aforward-link transponder 3430 coupled between a feeder-link constituentreceive element 3416 and a user-link constituent transmit element 3429.As described above, each forward uplink signal 521 is beam weighted(e.g., at a forward beamformer 529 in the CPS 505 of the ground segment502) prior to transmission from an AN 515. Each AN 515 receives a uniqueforward uplink signal 521 and transmits the unique forward uplink signal521 via one of M uplinks (e.g., in a time-synchronized manner). Theforward uplink signals 521 are received from geographically distributedlocations (e.g., from the ANs 515) by some or all of the forward-linktransponders 3430 in a superposed manner that creates composite inputforward signals 545. The forward-link transponders 3430 concurrentlyreceive respective composite input forward signals 545, though withslightly different timing due to differences in the locations of eachreceiving feeder-link constituent receive element 3416 associated witheach forward-link transponder 3430. For example, even though eachfeeder-link constituent receive element 3416 can receive a composite ofthe same plurality of forward uplink signals 521, the received compositeinput forward signals 545 can be slightly different. The composite inputforward signals 545 are received by L forward-link transponders 3430 viarespective feeder-link constituent receive elements 3416, communicatedthrough the L forward-link transponders 3430 to L correspondinguser-link constituent transmit elements 3429, and transmitted by the Luser-link constituent transmit elements 3429 to one or more of the Kuser beam coverage areas 519 (e.g., as forward downlink signals 522,each corresponding to a respective one of the received composite inputforward signals 545). In this way, the multiple forward-link signalpaths 3602 (e.g., forward-link transponders 3430) induce multipath inthe forward-link communications. As described above, there are L (or upto L) different ways for a signal to get from an AN 515 to a particularuser terminal 517 in a user beam coverage area 519. The end-to-end relay3403 thereby induces multiple (e.g., up to L) signal paths 3602 betweenone AN 515 and one user terminal 517 (or one user beam coverage area519), which may be referred to collectively as an end-to-end forwardmultipath channel 2208 (e.g., similar to FIG. 10 ).

The end-to-end forward multipath channels 2208 can be modeled in thesame manner described above. For example, Cr is the L×M forward uplinkradiation matrix, At is the K×L forward downlink radiation matrix, andEfwd is the L×L forward payload matrix for the paths from thefeeder-link constituent receive elements 3416 to the user-linkconstituent transmit elements 3429. In some cases, the forward payloadmatrix Efwd and return payload matrix Eret may be different to reflectdifferences between the forward-link signal paths 3602 and thereturn-link signal paths 3502. As described above, the end-to-endforward multipath channel from a particular AN 515 to a user terminal517 in a particular user beam coverage area 519 is the net effect of theL different signal paths induced by L unique forward-link signal paths3602 through the end-to-end relay 3403. With K user beam coverage areas519 and M ANs 515, there can be M×K induced end-to-end forward multipathchannels in the end-to-end forward link 501, and each can beindividually modeled to compute a corresponding element of an M×Kforward channel matrix Hfwd (At×Efwd×Cr). As noted with reference to thereturn direction, not all ANs 515, user beam coverage areas 519, and/orforward-link transponders 3430 have to participate in the end-to-endforward multipath channels. In some cases, the number of user beams K isgreater than the number of forward-link transponders 3430 L in thesignal path of the end-to-end forward multipath channel; and/or thenumber of ANs 515 M is greater than the number of forward-linktransponders 3430 L in the signal path of the end-to-end forwardmultipath channel. As described with reference to FIG. 5 , anappropriate beam weight may be computed for each of the plurality ofend-to-end forward multipath channels by the CPS 505 to form the forwarduser beams. Using multiple transmitters (ANs 515) to a single receiver(user terminal 517) can provide transmit path diversity to enable thesuccessful transmission of information to any user terminal 517 in thepresence of the intentionally induced multipath channel.

FIGS. 41-43 describe end-to-end relays 3403 implemented with separateforward-link transponders 3430 and return-link transponders 3440. FIGS.44A and 44B show an illustration of an example forward signal path 3700(like the forward signal path 3602 of FIG. 43 ) and return signal path3750 (like the return signal path 3502 of FIG. 42 ), respectively. Asdescribed above, the forward signal path 3700 includes a forward-linktransponder 3430 coupled between a feeder-link constituent receiveelement 3416 and a user-link constituent transmit element 3429. Thereturn signal path 3750 includes a return-link transponder 3440 coupledbetween a user-link constituent receive element 3426 and a feeder-linkconstituent transmit element 3419. In some cases, each forward-linktransponder 3430 and each return-link transponder 3440 is a cross-poletransponder.

FIG. 63A illustrates an example frequency spectrum allocation 6300 inaccordance with various embodiments of the present disclosure. Examplefrequency spectrum allocation 6300 of FIG. 63A illustrates two frequencyranges 6325 a and 6330 a. Though illustrated as being separated,frequency ranges 6325 a and 6330 a may alternatively be adjacent (e.g.,one contiguous range). As illustrated in FIG. 63A, the forward-linktransponder 3430 receives a forward uplink signal 6340 a (e.g., whichmay be an example of forward uplink signal 521 of FIG. 41 ) at an uplinkfrequency range 6330 a with left-hand circular polarization (LHCP) andoutputs a forward downlink signal 6345 a (e.g., which may be an exampleof forward downlink signal 522 of FIG. 41 ) at a downlink frequencyrange 6325 a with right-hand circular polarization (RHCP); and eachreturn-link transponder 3440 receives a return uplink signal 6350 a(e.g., which may be an example of return uplink signal 525 of FIG. 41 )at the uplink frequency range 6330 a with right-hand circularpolarization (RHCP) and outputs a return downlink signal 6355 a (e.g.,which may be an example of return downlink signal 527 of FIG. 41 ) atthe downlink frequency range 6325 a with left-hand circular polarization(LHCP). One such case (i.e., following the polarizations described inthe preceding example) is illustrated by following only the solid linesof FIGS. 44A and 44B, and another such case (i.e., following oppositepolarizations from those described in the preceding example) isillustrated by following only the dashed lines of FIGS. 44A and 44B.

In other cases, some or all transponders can provide a dual-pole signalpath pair. For example, following both the solid and dashed lines ofFIGS. 44A and 44B, the forward-link transponders 3430 and thereturn-link transponders 3440 can receive forward uplink signals 521 atthe same or different uplink frequency with both polarizations (LHCP andRHCP) and can both output forward downlink signals 522 at the same ordifferent downlink frequency with both polarizations (RHCP and LHCP).Such cases can use any suitable type of interference mitigationtechniques (e.g., using time division, frequency division, spatialseparation, etc.) and can enable multiple systems to operate inparallel. One such frequency-division implementation is shown in theexample frequency allocation 6301 of FIG. 63B. In example frequencyallocation 6301, each forward-link transponder 3430 receives a forwarduplink signal 6340 b over a first portion of uplink frequency range 6330b (e.g., using both polarizations) and outputs a forward downlink signal6345 b over a first portion of a downlink frequency range 6325 b (e.g.,using both polarizations); and each return-link transponder 3440receives a return uplink signal 6350 b over a second portion of theuplink frequency range 6330 b (e.g., using both polarizations) andoutputs a return downlink signal 6355 a over a second portion of thedownlink frequency range 6325 b (e.g., using both polarizations). Insome cases, the bandwidths of the first portions and second portions ofthe frequency ranges 6330 b and 6325 b may be equal. In other examples,the bandwidths of the first portions and second portions may bedifferent. As an example, when traffic flows through end-to-end relay3403 predominantly in the forward direction (represented by ETE forwardlink 501 in FIG. 41 ), the bandwidths of the first portions of frequencyranges 6330 b and 6325 b used for forward link communications may belarger (e.g., significantly larger) than the bandwidths of the secondportions used for return link communications.

In some cases, the end-to-end relay 3403 includes a large number oftransponders, such as 512 forward-link transponders 3430 and 512return-link transponders 3440 (e.g., 1,024 transponders total). Otherimplementations can include smaller numbers of transponders, such as 10,or any other suitable number. In some cases, the antenna elements areimplemented as full-duplex structures, so that each receive antennaelement shares structure with a respective transmit antenna element. Forexample, each illustrated antenna element can be implemented as two offour waveguide ports of a radiating structure adapted for bothtransmission and reception of signals. In some cases, only thefeeder-link elements, or only the user-link elements, are full duplex.Other implementations can use different types of polarization. Forexample, in some implementations, the transponders can be coupledbetween a receive antenna element and transmit antenna element of thesame polarity.

Both the example forward-link transponder 3430 and return-linktransponder 3440 can include some or all of LNAs 3705, frequencyconverters and associated filters 3710, channel amplifiers 3715, phaseshifters 3720, power amplifiers 3725 (e.g., traveling wave tubeamplifiers (TWTAs), solid state power amplifiers (SSPAs), etc.) andharmonic filters 3730. In dual-pole implementations, as shown, each polehas its own signal path with its own set of transponder components. Someimplementations can have more or fewer components. For example, thefrequency converters and associated filters 3710 can be useful in caseswhere the uplink and downlink frequencies are different. As one example,each forward-link transponder 3430 can accept an input at a firstfrequency range and can output at a second frequency range; and eachreturn-link transponder 3440 can accept an input at the first frequencyrange and can output at the second frequency range.

In some cases, multiple sub-bands are used (e.g., seven 500 MHzsub-bands, as described above). For example, in some cases, transponderscan be provided that operate over the same sub-bands as used in amultiple sub-band implementation of the ground network, effectively toenable multiple independent and parallel end-to-end beamforming systemsthrough a single end-to-end relay (each end-to-end beamforming systemoperating in a different sub-band). In such cases, each transponder caninclude multiple frequency converters and associated filters 3710,and/or other components, dedicated to handling one or more of thesub-bands. The use of multiple frequency sub-bands may allow relaxedrequirements on the amplitude and phase response of the transponder, asthe ground network may separately determine beam weights used in each ofthe sub-bands, effectively calibrating out passband amplitude and phasevariation of the transponders. For example, with separate forward andreturn transponders, and using 7 sub-bands, a total of 14 different beamweights may be used for each beam (i.e., 7 sub-bands*2 directions(forward and return)). In other cases, a wide bandwidth end-to-endbeamforming system may use multiple sub-bands in the ground network butpass one or more (or all) sub-bands through wideband transponders (e.g.,passing 7 sub-bands, each 500 MHz wide, through a 3.5 GHz bandwidthtransponders). In some cases, each transponder path includes only a LNA3705, a channel amplifier 3715, and a power amplifier 3725. Someimplementations of the end-to-end relay 3403 include phase shiftcontrollers and/or other controllers that can individually set thephases and/or other characteristics of each transponder as describedabove.

The antenna elements can transmit and/or receive signals in any suitablemanner. In some cases, the end-to-end relay 3403 has one or more arrayfed reflectors. For example, the feeder-link antenna subsystem 3410 canhave a feeder-link reflector for both transmit and receive, or aseparate feeder-link transmit reflector and feeder-link receivereflector. In some cases, the feeder-link antenna subsystem 3410 canhave multiple feeder-link reflectors for transmission or reception, orboth. Similarly, the user-link antenna subsystem 3420 can have auser-link reflector for both transmit and receive, or a separateuser-link transmit reflector and user-link receive reflector. In somecases, the user-link antenna subsystem 3420 can have multiple user-linkreflectors for transmission or reception, or both. In one example case,the feeder-link antenna subsystem 3410 comprises an array of radiatingstructures, and each radiating structure includes a feeder-linkconstituent receive element 3416 and a feeder-link constituent transmitelement 3419. In such a case, the feeder-link antenna subsystem 3410 canalso include a feeder-link reflector that illuminates the feeder-linkconstituent receive elements 3416 and is illuminated by the feeder-linkconstituent transmit elements 3419. In some cases, the reflector isimplemented as multiple reflectors, which may be of different shapes,sizes, orientations, etc. In other cases, the feeder-link antennasubsystem 3410 and/or the user-link antenna subsystem 3420 isimplemented without reflectors, for example, as a direct radiatingarray.

As discussed above, achieving a relatively uniform distribution of ANs515 across a given user coverage area 3460 may involve placing ANs 515in undesirable locations. Thus, the present disclosure describestechniques to enable the ANs 515 to be geographically distributed withinan AN area 3450 that is smaller (sometimes significantly) than the usercoverage area 3460. For example, in some cases the AN area 3450 may beless than half, less than one quarter, less than one-fifth, or less thanone-tenth the physical area of the user coverage area 3460. In addition,multiple AN areas 3450 may be used concurrently or may be activated foruse at different times. As discussed herein, these techniques includethe use of different sized reflectors, compound reflector(s),selectively coupled transponders, different user link and feeder linkantenna subsystems, etc.

As noted above, separating the feeder-link antenna subsystem 3410 andthe user-link antenna subsystem 3420 can enable servicing of one or moreAN areas 3450 that are distinct from one or more user coverage areas3460. For example, the feeder-link antenna subsystem 3410 can beimplemented with a reflector having an appreciably larger physical areathan the reflector of the user coverage area 3460. The larger reflectorcan permit a large number of ANs 515 to be geographically distributed inan appreciably smaller AN area 3450, such as in a small subset of theuser coverage area 3460. Some examples are shown in FIGS. 45A-45G.Alternatively, an AN area 3450 that is a subset of the user coveragearea may be deployed using a single antenna subsystem for both thefeeder-link and user-link by using different frequency ranges for thefeeder-link and user-links. For example, an AN area 3450 that isone-quarter the area of a user coverage area 3460 may be deployed usinga feeder-link carrier frequency that is approximately double theuser-link carrier frequency. In one example, the user-link may use afrequency range (or ranges) in the K/Ka bands (e.g., around 30 GHz)while the feeder-link uses frequency range(s) in the V/W bands (e.g.,around 60 GHz). In this case, the AN area 3450 will be concentric withthe user coverage area 3460.

FIG. 45A shows an example of an end-to-end relay 3403 (e.g., asatellite) visible Earth coverage area 3800. In the example end-to-endrelay 3403, the feeder-link antenna subsystem 3410 includes an 18-meterfeeder-link reflector, and the user-link antenna subsystem 3420 includesa 2-meter user-link reflector (e.g., the feeder-link reflector area isabout eighty times larger than the user-link reflector area). Eachantenna subsystem also includes an array of 512 cooperating constituentreceive/transmit elements. The example end-to-end relay 3403 can include512 forward-link transponders 3430 (e.g., forming 512 forward signalpaths 3700 as shown in FIG. 44A) and 512 return-link transponders 3440(e.g., forming 512 return signal paths 3750 as shown in FIG. 44B). Froma geostationary orbital position of the end-to-end relay 3403, theuser-link antenna subsystem 3420 illuminates user coverage area 3460that extends substantially over the visible Earth coverage area 3800while the feeder-link reflector illuminates AN area 3450 that is afraction of the user coverage area 3460. Although the AN area 3450 is asmall subset of the large user coverage area 3460, a large systemcapacity including a large number of user beams can be supported usingend-to-end beamforming with a large number of ANs 515 in the AN area3450 (e.g., used cooperatively in an AN cluster). For example, hundredsof cooperating ANs 515 may be geographically distributed within AN area3450 shown in FIG. 45A as a shaded region in the eastern United States.In one example, 597 ANs 515 are geographically distributed within ANarea 3450.

FIG. 46A shows the visible earth coverage with end-to-end beamformingapplied between the ANs 515 in the AN area 3450 and the user coveragearea 3460. The user coverage area 3460 includes 625 user beam coverageareas 519 providing service to user terminals 517 within the visibleEarth coverage area 3800.

FIG. 45B shows an example of an end-to-end relay 3403 (e.g., asatellite) Continental United States (CONUS) coverage area 3900. Theexample end-to-end relay 3403 is similar to the example shown in FIG.45A, except that the feeder-link antenna subsystem 3410 uses an 18-meterfeeder-link reflector while the user-link antenna subsystem 3420includes a 5-meter user-link reflector (e.g., the area of thefeeder-link reflector is about thirteen times larger than the area ofthe user-link reflector). The AN area 3450 (e.g., the area containingthe cooperating AN cluster) is the same as that of FIG. 45A: a regionthat is a small subset of the user coverage area 3460 in the easternUnited States having e.g., 597 ANs 515 distributed therein.

FIG. 46B shows the CONUS coverage area 3900 with end-to-end beamformingapplied between the ANs 515 in the AN area 3450 and the user coveragearea 3460. The user coverage area 3460 includes 523 user beam coverageareas 519 providing service to user terminals 517 within the CONUScoverage area.

Various geographical and relative locations of the AN cluster aresupported by the present disclosure. As described herein, an end-to-endrelay 3403 like those illustrated in FIGS. 49A and 49B can providecommunications service between one or more user coverage areas 3460 andANs 515 located in one or more AN areas 3450. In some examples, such asthe example illustrated in FIG. 45B, the AN area 3450 may overlap or belocated entirely within the user coverage area 3460. Additionally oralternatively, an AN area 3450 may be non-overlapping with a usercoverage area 3460 as illustrated in FIG. 45C. In some cases, such anarrangement may require the use of a special loopback mechanism, whichis discussed below with reference to FIGS. 55A-55C.

As another example of a possible geographic arrangement, the AN cluster(e.g., the AN area 3450) may at least partially overlap with a lowdemand area of the user coverage area 3460. An example is shown in FIG.45D, where the AN area 3450 is located in a low demand area of usercoverage area 3460. In some cases, a low demand area may be determinedbased on the demand for the communication service being below a demandthreshold. For example, the low demand area may have an average demandthat is less than a fraction (e.g., one-half, one-quarter, etc.) of theaverage demand across other served areas of user coverage area 3460.Such a deployment may support increased system capacity in higher demandareas (e.g., by allowing portions of the frequency spectrum associatedwith feeder-link communications in the low demand area to be used foruser beams in the higher demand areas). That is, a given systembandwidth (which may be a contiguous or multiple non-contiguousfrequency ranges) may be mostly or fully utilized for serving user beamsin areas outside the low demand area and may be allocated mostly tofeeder-link communications within the low demand area, with the userbeams in the low demand area being allocated a smaller portion (e.g.,less than half) of the system bandwidth. Thus, in some cases, theuser-link communications in higher demand areas may use at least aportion of the same frequency bandwidth used for feeder-linkcommunications in a low demand area in which the access node area 3450is located. In this example, the AN area 3450 is contained completelywithin user coverage area 3460, although the two may only partiallyoverlap in some cases.

In some cases, the AN cluster may be located within (e.g., on thesurface of) an aquatic body (e.g., a lake, sea, or ocean). An example isshown in FIG. 45E, which shows a user coverage area 3460 including theUnited States and an AN area 3450 located off the eastern coast of theUnited States. In some cases, the AN area may at least partially overlapwith a landmass (e.g., some ANs 515 may not be located within theaquatic body). Thus, the example discussed with respect to FIG. 45Eincludes a scenario in which only one AN 515 is located within theaquatic body, all ANs 515 are located within the aquatic body, or someintermediate number of ANs 515 are located within the aquatic body.Benefits of locating parts or all of an AN cluster on an aquatic bodyinclude availability of large areas for the AN cluster in proximity toland masses where user coverage is desired, flexibility in placement ofANs 515 within the AN area 3450, and reduced competition for spectrumrights. For example, regulatory considerations such as interference andband-sharing with other services may be reduced when an AN cluster isnot located over a particular country or landmass.

ANs 515 located within the aquatic body may be located on fixed orfloating platforms. Examples of fixed platforms used for ANs 515 includefixed oil platforms, fixed offshore wind turbines, or other platformsinstalled on pilings. Examples of floating platforms include barges,buoys, offshore oil platforms, floating offshore wind turbines, and thelike. Some fixed or floating platforms may already have power sources,while other fixed or floating platforms dedicated for use in an ANcluster may be configured with power generation (e.g., a generator,solar power generation, wind turbine, etc.). Distribution of access nodespecific forward signals 521 from a beamformer 529 to the ANs 515 andcomposite return signals 1706 from the ANs 515 to the beamformer 531 maybe provided via a distribution network 518 that includes wired orwireless links between the beamformer(s) or a distribution platform andthe ANs 515. In some cases, the distribution network 518 may include asubmarine cable coupled with the beamformer(s) and ANs 515 distributedwithin the aquatic body as discussed with reference to FIG. 45G. Thesubmarine cable may also provide a power source. The distributionnetwork may additionally or alternatively include wireless RF links(e.g., microwave backhaul links) or free space optical links. In someexamples, the beamformer(s), a distribution point for the beamformer(s),or the distribution network 518 as a whole may be located within theaquatic body. For example, FIG. 58 shows a CPS 505 disposed on anoffshore (e.g., fixed or floating) platform 5805 that communicatestraffic to a terrestrial network node and is coupled to ANs 515 in theaquatic body via distribution network 518.

In some cases, at least some ANs 515 in the AN cluster may be mobile(e.g., may be located on moveable platforms). For example, ANs 515within an aquatic body may be located on boats or barges that may becontrolled to relocate position as illustrated by floating platform 5805in FIG. 58 . Similarly, terrestrial ANs 515 may be located on vehicularplatforms while airborne ANs 515 may be located on mobile platforms suchas aircraft, balloons, drones, and the like. In some examples, mobileANs 515 may be used to optimize distribution of ANs 515 within the ANarea 3450. For example, ANs 515 may be relocated for better geographicdistribution within the AN area 3450, or ANs 515 may be relocated uponfailure of one or more ANs 515 (e.g., to redistribute the available ANs515). The beamforming weights may be recalculated for the new positionsand the ANs 515 may resynchronize transmit timing and phase to adjust tothe new positions, as described above.

In some examples, the AN area 3450 may be relocated using mobile ANs 515(e.g., one or more ANs 515 in the AN cluster may be located on mobileplatforms). An example is shown in FIG. 45F, which shows an initial ANarea 3450 a including multiple ANs 515 geographically distributed withinthe AN area 3450 a. For various reasons, the AN cluster may be relocatedto be within new AN area 3450 b. For example, a mobile AN cluster may beused to adapt to changes in position of the end-to-end relay 3403. Inone example, an orbital position or orientation of a satelliteend-to-end relay 3403 changes due to a change in deployment to a neworbital slot or because of orbital drift or alignment, and the change inAN area 3450 adapts to the new orbital position or orientation. Themobile ANs 515 may move to new positions within the new AN area 3450 b.Additionally, while the mobile AN cluster is displayed as being locatedwithin an aquatic body, some or all of the ANs 515 may be located onland (e.g., mobile ANs 515 need not be located in an aquatic body). Insome cases, one or more of the ANs 515 may be located on an airbornecraft (e.g., a plane, a balloon, a drone, etc.). Also, while the currentexample describes first and second AN areas 3450 a and 3450 b that aresimilar in size at different locations, the AN areas 3450 at thedifferent locations may be (e.g., significantly) different (e.g., due toa difference in slant range or adaptation of an antenna assembly on theend-to-end relay). As an example, the first and second AN areas 3450 aand 3450 b may have the same (or similar) center points butsignificantly different physical sizes (e.g., through a combination oforbit slot shift and repointing of the end-to-end relay antenna).

As an example, the AN cluster may initially be located at a firstlocation 3450 a. While at the first location 3450 a, each AN 515 of theAN cluster may receive an access node-specific forward signal fortransmission via end-to-end relay 3403 to one or more of the userterminals in user coverage area 3460. In aspects, the accessnode-specific forward signal may be received from a forward beamformer529 via a distribution network 518, which may be a free space opticallink or any other suitable link. As discussed above, the accessnode-specific forward signals may be appropriately weighted by theforward beamformer 529 before reception at the AN 515. While at thefirst location 3450 a, each AN 515 may synchronize a forward uplinksignal 521 for reception at the end-to-end relay 3403 so that theforward uplink signal 521 is time and phase aligned with other forwarduplink signals 521 from other ANs 515 in the AN cluster. Synchronizationmay be accomplished using any of the techniques described herein (e.g.,using relay beacons).

Subsequently, the AN cluster (or portions thereof) may move to a secondlocation 3450 b. The movement may be in response to some stimulus (e.g.,a change in location of the end-to-end relay, weather patterns, etc.).At the second location 3450 b, the ANs 515 of the AN cluster may obtainweighted access node-specific forward signals (e.g., generated using anupdated beam weight matrix determined based on the new locations of theANs 515 within the new AN area 3450 b), synchronize transmissions, andtransmit forward uplink signals 521 to end-to-end relay 3403. Whiledescribed as being performed at the second location, one or more ofthese steps may be performed prior to reaching the second location.

In some cases, the location and shape of the AN cluster may beconfigured to take advantage of existing network infrastructure. Forexample, as shown in FIG. 45G, the AN area 3450 may be located near anexisting submarine cable 4551 (e.g., fiber-optic cable used in Internetbackbone communications, etc.). The submarine cable 4551 may alsoprovide a power source. The distribution network 518 (e.g., between ANs)may additionally or alternatively include wireless RF links (e.g.,microwave backhaul links) or free space optical links. In some examples,the beamformer(s), a distribution point for the beamformer, or thedistribution network 518 as a whole may be located within the aquaticbody. As shown in FIG. 45G, one or more of the AN areas 3450 may beshaped (e.g., using an appropriately shaped reflector, etc.) so as tominimize the total distance between the ANs 515 and the submarine cable4551. The example of FIG. 45G shows an elliptically shaped AN area 3450,though any suitable shape may be used. Further, while only one AN area3450 is displayed in FIG. 45G, multiple AN areas 3450 may exist (e.g.,located along the same submarine cable 4551 or different submarinecables 4551). The multiple AN areas 3450 may be disjoint or overlap atleast partially.

Multiple Coverage Areas

In the example end-to-end relays 3403 described above, the user-linkantenna subsystem 3420 is described as a single antenna subsystem (e.g.,with a single user-link reflector), and the feeder-link antennasubsystem 3410 is described as a single antenna subsystem (e.g., with asingle feeder-link reflector). In some cases, the user-link antennasubsystem 3420 can include one or more antenna subsystems (e.g., two ormore sub-arrays of constituent antenna elements) associated with one ormore user-link reflectors, and the feeder-link antenna subsystem 3410can include one or more antenna subsystems associated with one or morefeeder-link reflectors. For example, some end-to-end relays 3403 canhave a user-link antenna subsystem 3420 that includes a first set ofuser-link constituent receive/transmit elements associated with a firstuser-link reflector (e.g., each element is arranged to illuminate,and/or be illuminated by, the first user-link reflector) and a secondset of user-link constituent receive/transmit elements associated with asecond user-link reflector. In some cases, the two user-link reflectorsare approximately the same physical area (e.g., within 5%, 10%, 25%,etc.) of each other. In some cases, one user-link reflector issignificantly larger (e.g., 50% larger, at least twice the physicalarea, etc.) than the other. Each set of the user-link constituentreceive/transmit elements, and its associated user-link reflector, canilluminate a corresponding, distinct user coverage area 3460. Forexample, the multiple user coverage areas can be non-overlapping,partially overlapping, fully overlapping (e.g., a smaller user coveragecould be contained within a larger user coverage area), etc. In somecases, the multiple user coverage areas can be active (illuminated) atthe same time. Other cases, as described below, can enable selectiveactivation of the different portions of user-link constituentreceive/transmit elements, thereby activating different user coverageareas at different times. Similarly, selective activation of differentportions of feeder-link constituent receive/transmit elements canactivate different AN areas 3450 at different times. Switching betweenmultiple coverage areas may be coordinated with the CPS 505. Forexample, beamforming calibration, beam weight calculation and beamweight application may occur in two parallel beamformers, one for eachof two different coverage areas. The usage of appropriate weights in thebeamformers can be timed to correspond to the operation of theend-to-end relay. For example, switching between multiple coverage areasmay be coordinated to occur at a time-slice boundary if time-slicebeamformers are employed.

FIGS. 47A and 47B show an example forward signal path 4000 and returnsignal path 4050, respectively, each having selective activation ofmultiple user-link antenna subsystems 3420. Forward signal path 4000(and other forward signal paths described herein) may be an example offorward signal path 3602 described with reference to FIG. 43 . Returnsignal path 4050 (and other return signal paths described herein) may bean example of return signal path 3502 described with reference to FIG.42 . For example, each forward signal path 4000 may have a transponder3430 coupled between constituent antenna elements. In FIG. 47A, theforward-link transponder 3430 b is similar to the one described withreference to FIG. 44A, except that the output side of the forward-linktransponder 3430 b is selectively coupled to one of two user-linkconstituent transmit elements 3429, each part of a separate user-linkantenna subsystem 3420 (e.g., each part of a separate array 3425 ofcooperating user-link constituent transmit elements 3429). As describedabove, the forward-link transponder 3430 b can include some or all ofLNAs 3705 a, frequency converters and associated filters 3710 a, channelamplifiers 3715 a, phase shifters 3720 a, power amplifiers 3725 a, andharmonic filters 3730 a.

The forward-link transponder 3430 b of FIG. 47A further includesswitches 4010 a (forward-link switches) that selectively couple thetransponder either to a first user-link constituent transmit element3429 a (of a first user-link antenna element array 3425 a) via a firstset of power amplifiers 3725 a and harmonic filters 3730 a, or to asecond user-link constituent transmit element 3429 b (of a seconduser-link antenna element array 3425 b) via a second set of poweramplifiers 3725 a and harmonic filters 3730 a. For example, in a firstswitch mode, the forward-link transponder 3430 b effectively forms asignal path between a feeder-link constituent receive element 3416 and afirst user-link constituent transmit element 3429 a: and in a secondswitch mode, the forward-link transponder 3430 b effectively forms asignal path between the same feeder-link constituent receive element3416 and a second user-link constituent transmit element 3429 b. Theswitches 4010 a can be implemented using any suitable switching means,such as an electromechanical switch, a relay, a transistor, etc. Thoughshown as switches 4010 a, other implementations can use any othersuitable means for selectively coupling the input of the forward-linktransponder 3430 to multiple outputs. For example, the power amplifiers3725 a can be used as switches (e.g., providing high gain when “on,” andzero gain (or loss) when “off”). Switches 4010 a may be examples ofswitches that selectively couple one input to one of two or moreoutputs.

In FIG. 47B, the return-link transponder 3440 b functionally mirrors theforward-link transponder 3430 of FIG. 47A. Rather than selectivelycoupling the output side of the transponder, as in the forward-link caseof FIG. 47A, the input side of the return-link transponder 3440 b isselectively coupled to one of two user-link constituent receive elements3426. Again, each user-link constituent receive element 3426 can be partof a separate array of cooperating user-link constituent receiveelements 3426, which may be part of the same user-link antenna subsystem3420, or different user-link antenna subsystems 3420). As describedabove (e.g., in FIG. 44B), the return-link transponder 3440 can includesome or all of LNAs 3705 b, frequency converters and associated filters3710 b, channel amplifiers 3715 b, phase shifters 3720 b, poweramplifiers 3725 b, and harmonic filters 3730 b.

The return-link transponder 3440 b of FIG. 47B further includes switches4010 b (return-link switches) that selectively couple the transpondereither to a first user-link constituent receive element 3426 a (of afirst user-link antenna element array 3425 a) via a first set of LNAs3705 b, or to a second user-link constituent receive element 3426 b (ofa second user-link antenna element array 3425 b) via a second set ofLNAs 3705 b. For example, in a first switch mode, the return-linktransponder 3440 b effectively forms a signal path between a firstuser-link constituent receive element 3426 a and a feeder-linkconstituent transmit element 3419; and in a second switch mode, thereturn-link transponder 3440 b effectively forms a signal path between asecond user-link constituent receive element 3426 b and the samefeeder-link constituent transmit element 3419. The switches 4010 b canbe implemented using any suitable switching means, such as anelectromechanical switch, a relay, a transistor, etc. Though shown asswitches 4010 b, other implementations can use any other suitable meansfor selectively coupling the output of the forward-link transponder 3440b to multiple inputs. For example, the power amplifiers 3705 b can beused as switches (e.g., providing high gain when “on,” and zero gain (orloss) when “off”). Switches 4010 b may be examples of switches thatselectively couple one of two or more inputs to a single output.

Examples of the end-to-end relay 3403 can include a switch controller4070 to selectively switch some or all of the switches 4010 (or othersuitable selective coupling means) according to a switching schedule.For example, the switching schedule can be stored in a storage deviceon-board the end-to-end relay 3403. In some cases, the switchingschedule effectively selects which user-link antenna element array 3425to activate (e.g., which set of user beams to illuminate) in each of aplurality of time intervals (e.g., timeslots). In some cases, theswitching allocates equal time to the multiple user-link antenna elementarrays 3425 (e.g., each of two arrays is activated for about half thetime). In other cases, the switching can be used to realizecapacity-sharing goals. For example, one user-link antenna element array3425 can be associated with higher-demand users and can be allocated agreater portion of time in the schedule, while another user-link antennaelement array 3425 can be associated with lower-demand users and can beallocated a smaller portion of time in the schedule.

FIGS. 48A and 48B show an example of end-to-end relay 3403 coverageareas 4100 and 4150 that include multiple, selectively activated usercoverage areas 3460 a and 3460 b, respectively. The example end-to-endrelay 3403 is similar to the relay in FIGS. 38 and 39 except for thepresence of different antenna subsystems. In this example, the user-linkantenna subsystem 3420 includes two 9-meter user-link reflectors, andthe transponders are configured to selectively activate only half of theuser beam coverage areas 519 at any given time (e.g., the transpondersare implemented as in FIGS. 47A and 47B). For example, during a firsttime interval, as shown in FIG. 48A, the user coverage area 3460 aincludes 590 active user beam coverage areas 519. The active user beamcoverage areas 519 effectively cover the western half of the UnitedStates. The AN area 3450 (the AN cluster) is the same as that of FIGS.38 and 39 : a region in the eastern United States having e.g., 597 ANs515 distributed therein. During the first time interval, the AN area3450 does not overlap with the active user coverage area 3460 a. Duringa second time interval, as shown in FIG. 48B, the user coverage area3460 b includes another 590 active user beam coverage areas 519. Theactive user beam coverage areas 519 in the second time intervaleffectively cover the eastern half of the United States. The AN area3450 does not change. However, during the second time interval, the ANarea 3450 is fully overlapped by (is a subset of) the active usercoverage area 3460 b. Capacity may be flexibly allocated to variousregions (e.g., between eastern and western user coverage areas 3460) bydynamically adjusting the ratio of time allocated to the correspondinguser-link antenna sub-systems 3420.

While the previous example illustrates two similarly sized user coverageareas 3460, other numbers of user coverage areas 3460 can be provided(e.g., three or more) and can be of differing sizes (e.g., earthcoverage, continental U.S. only, U.S. only, regional only, etc.). Incases with multiple user coverage areas 3460, the user coverage areas3460 can have any suitable geographic relationship. In some cases, firstand second user coverage areas 3460 partially overlap (e.g., as shown inFIGS. 48A and 48B). In other cases, a second user coverage area 3460 canbe a subset of a first user coverage area 3460 (e.g., as shown in FIGS.46A and 46B). In other cases, the first and second user coverage areas3460 do not overlap (e.g., are disjoint).

In some cases, it can be desirable for traffic of particular geographicregions to terminate in their respective regions. FIG. 50A illustrates afirst AN area 3450 a in North America used to provide communicationsservice to a first user coverage area 3460 a in North America, and asecond AN area 3450 b to provide communications service to a second usercoverage area 3460 b in South America. In some cases, the ANs within thefirst AN area 3450 a exchange signals with a first CPS (e.g., locatedwithin or proximate to AN area 3450 a), and the ANs within the second ANarea 3450 b exchange signals with a second CPS (e.g., located within orproximate to AN area 3450 b) that is separate and distinct from thefirst CPS. For example, the first AN The end-to-end relay 3403 as shownin FIGS. 49A and 49B may support multiple user coverage areas withmultiple AN areas as illustrated in FIG. 50A. Each combination of ANarea and user coverage area may employ frequency allocations 6300 or6301 as shown in FIG. 63A or 63B.

FIG. 49A shows an example forward signal path 4900 of an end-to-endrelay 3403 for supporting multiple user coverage areas with multiple ANareas 3450. The example forward signal path 4900 has a firstforward-link transponder 3430 c coupled between a first feeder-linkconstituent receive element 3416 a of a first feeder-link antennaelement array 3415 a and a first user-link constituent transmit element3429 a of a first user-link antenna element array 3425 a. In addition,the example forward signal path 4900 has a second forward-linktransponder 3430 c coupled between a second feeder-link constituentreceive element 3416 b of a second feeder-link antenna element array3415 b and a second user-link constituent transmit element 3429 b of asecond user-link antenna element array 3425 b. As described above, eachof the forward-link transponders 3430 can include some or all of LNAs3705 a, frequency converters and associated filters 3710 a, channelamplifiers 3715 a, phase shifters 3720 a, power amplifiers 3725 a, andharmonic filters 3730 a.

FIG. 49B shows an example return signal path 4950 of an end-to-end relay3403 for supporting multiple user coverage areas with multiple AN areas3450. The example return signal path 4950 has a first return-linktransponder 3440 c coupled between a first user-link constituent receiveelement 3426 a of a first user-link antenna element array 3425 a and afirst feeder-link constituent transmit element 3419 a of a firstfeeder-link antenna element array 3415 a. In addition, the examplereturn signal path 4950 has a second return-link transponder 3440 ccoupled between a second user-link constituent receive element 3426 b ofa second user-link antenna element array 3425 b and a second feeder-linkconstituent transmit element 3419 b of a second feeder-link antennaelement array 3415 b. As described above, each of the return-linktransponders 3440 can include some or all of LNAs 3705 b, frequencyconverters and associated filters 3710 b, channel amplifiers 3715 b,phase shifters 3720 b, power amplifiers 3725 b, and harmonic filters3730 b.

In some cases, feeder-link antenna element arrays 3415 a and 3415 b arepart of separate feeder-link antenna subsystems 3410. Alternatively, asingle feeder-link antenna subsystem 3410 may include both feeder-linkantenna element arrays 3415 a and 3415 b (e.g., via use of a singlereflector as described in more detail below with reference to FIGS. 56Aand 56B). Similarly, user-link antenna element arrays 3425 a and 3425 bmay be part of the same or separate user-link antenna subsystems 3420.The forward signal path 4900 and return signal path 4950 of FIGS. 49Aand 49B may be used to support multiple independent end-to-endbeamforming systems using a single end-to-end relay payload. Forexample, end-to-end beamforming between the first AN area 3450 a and thefirst user coverage area 3460 a shown in FIG. 50A may be supported byone beamformer and distribution system, while a separate and independentbeamformer and distribution system supports end-to-end beamformingbetween the second AN area 3450 b and the second user coverage area 3460b. FIGS. 49A and 49B illustrate examples where the constituent receiveelements may be the same as the constituent transmit elements, andtherefore only show one polarization in each direction. However, otherexamples may employ different constituent receive elements andconstituent transmit elements and may use multiple polarizations in eachdirection.

FIGS. 47A and 47B describe signal path selection on the user-link side.However, some cases alternatively or additionally include signal pathswitching on the feeder-link side. FIG. 51A shows an example forwardsignal path 5100 having selective activation of multiple user-linkantenna element arrays 3425 (which may be part of the same or differentuser-link antenna subsystems 3420) and multiple feeder-link antennaelement arrays 3415 (which may be part of the same or differentfeeder-link antenna subsystems 3410). The signal path has a forward-linktransponder 3430 d coupled between constituent antenna elements. Asdescribed above, the forward-link transponder 3430 d can include some orall of LNAs 3705 a, frequency converters and associated filters 3710 a,channel amplifiers 3715 a, phase shifters 3720 a, power amplifiers 3725a, and harmonic filters 3730 a. The input side of the forward-linktransponder 3430 d is selectively coupled to one of two feeder-linkconstituent receive elements 3416 (e.g., using switches 4010 b or anyother suitable path selection means). Each feeder-link constituentreceive element 3416 can be part of a separate feeder-link antennaelement array 3415 (e.g., each part of a separate array of cooperatingfeeder-link constituent receive elements 3416). The output side of theforward-link transponder 3430 d is selectively coupled to one of twouser-link constituent transmit elements 3429 (e.g., using switches 4010a or any other suitable path selection means). Each user-linkconstituent transmit element 3429 can be part of a separate user-linkantenna element array 3425 (e.g., each part of a separate array ofcooperating user-link constituent transmit elements 3429). One or moreswitching controllers 4070 (not shown) can be included in the end-to-endrelay 3403 for selecting between some or all of the four possible signalpaths enabled by the forward-link transponder 3430 d. For example, theswitching controller 4070 may operate the forward link transponder 3430d according to one of several switch modes, which may be determinedaccording to which AN areas 3450 are used to support user coverage areas3460. In one example, the switching controller 4070 applies a firstswitch mode for switches 4010 to couple the forward link transponders3430 d between the first feeder-link antenna element array 3415 a andthe first user-link antenna element array 3425 a, and applies secondswitch mode for switches 4010 to couple the forward link transponders3430 d between the second feeder-link antenna element array 3415 b andthe second user-link antenna element array 3425 b. Alternatively, afirst switch mode for switches 4010 may couple the forward linktransponders 3430 d between the first feeder-link antenna element array3415 a and the second user-link antenna element array 3425 b, and asecond switch mode for switches 4010 may couple the forward linktransponders 3430 d between the second feeder-link antenna element array3415 b and the first user-link antenna element array 3425 a.

FIG. 51B shows an example return signal path 5150 having selectiveactivation of multiple user-link antenna element arrays 3425 (e.g.,which may be part of the same or different user-link antenna subsystems3420) and multiple feeder-link antenna element arrays 3415 (e.g., whichmay be part of the same or different feeder-link antenna subsystems3410). The signal path has a return-link transponder 3440 d coupledbetween constituent antenna elements. As described above, thereturn-link transponder 3440 d can include some or all of LNAs 3705 b,frequency converters and associated filters 3710 b, channel amplifiers3715 b, phase shifters 3720 b, power amplifiers 3725 b, and harmonicfilters 3730 b. The input side of the return-link transponder 3440 d isselectively coupled to one of two user-link constituent receive elements3426 a, 3426 b (e.g., using switches 4010 b or any other suitable pathselection means). Each user-link constituent receive element 3426 a,3426 b can be part of a separate user-link antenna element array 3425 a,3425 b (e.g., each part of a separate array of cooperating user-linkconstituent receive elements 3426). The output side of the return-linktransponder 3440 d is selectively coupled to one of two feeder-linkconstituent transmit elements 3419 a or 3419 b (e.g., using switches4010 a or any other suitable path selection means). Each feeder-linkconstituent transmit element 3419 a or 3419 b can be part of a separatefeeder-link antenna element array 3415 a or 3415 b (e.g., each part of aseparate array of cooperating feeder-link constituent transmit elements3419). One or more switching controllers 4070 (not shown) can beincluded in the end-to-end relay 3403 for selecting between some or allof the four possible signal paths enabled by the return-link transponder3440 d. For example, the switching controller 4070 may operate thereturn-link transponder 3440 d according to one of several switch modes,which may be determined according to which AN areas 3450 are used tosupport user coverage areas 3460. In one example, the switchingcontroller 4070 applies a first switch mode for switches 4010 to couplethe return-link transponders 3440 d between the first user-link antennaelement array 3425 a and the first feeder-link antenna element array3415 a, and applies second switch mode for switches 4010 to couple thereturn-link transponders 3440 d between the second user-link antennaelement array 3425 b and the second feeder-link antenna element array3415 b. Alternatively, a first switch mode for switches 4010 may couplethe return-link transponders 3440 d between the first user-link antennaelement array 3425 a and the second feeder-link antenna element array3415 b, and a second switch mode for switches 4010 may couple thereturn-link transponders 3440 d between the second user-link antennaelement array 3425 b and the first feeder-link antenna element array3415 a.

The transponders of FIGS. 47A, 47B, 51A, and 51B are intended only toillustrate a few of many possible cases of end-to-end relays 3403employing path selection. Further, some cases can include path selectionbetween more than two user-link antenna element arrays 3425 or user-linkantenna subsystems 3420 and/or more than two feeder-link antenna elementarrays 3415 or feeder-link antenna subsystems 3410.

The end-to-end relay 3403 as shown in FIGS. 51A and 51B may supportmultiple user coverage areas 3460 with multiple AN areas 3450. Asdiscussed above, it can be desirable for traffic of particulargeographic regions to terminate in their respective regions. Forexample, an end-to-end relay 3403 with or without paired transponderslike those illustrated in FIGS. 51A and 51B can utilize a first AN area3450 a in North America to provide communications service to a firstuser coverage area 3460 a in North America and utilize a second AN area3450 b to provide communications service to a second user coverage area3460 b in South America as illustrated in FIG. 50A. Using path selection(e.g., switching) in the transponders, a single end-to-end relay 3403(e.g., a single satellite) can service traffic associated with the NorthAmerican user coverage area 3460 a using ANs 515 in the North AmericanAN area 3450 a (or using ANs 515 in the South American AN area 3450 b),and service traffic associated with the South American user coveragearea 3460 b using ANs 515 in the South American AN area 3450 b (or usingANs 515 in the North American AN area 3450 a). Capacity may be flexiblyallocated to various regions (e.g., between North and South Americanuser coverage areas 3460) by dynamically adjusting the ratio of timeallocated to the corresponding antenna sub-systems.

FIG. 50B illustrates a second possible deployment having multiple ANareas 3450 and multiple user coverage areas 3460. For example, thedeployment shown in FIG. 50B may be supported by the end-to-end relay3403 illustrated by FIGS. 51A and 51B. As shown in FIG. 50B, anend-to-end relay 3403 with path selection in the transponders servicestraffic in a first user coverage area 3460 a with a first AN area 3450 aand services traffic in a second user coverage area 3460 b with a secondAN area 3450 b. Because the first AN area 3450 a does not overlap withthe first user coverage area 3460 a, the same or overlapping portions ofbandwidth may be used for uplink or downlink communications between theend-to-end relay 3403 and user terminals or ANs. Additionally, in thepresent example, because AN area 3450 a or 3450 b and its correspondinguser coverage area 3460 a or 3460 b, respectively, do not overlap, aspecial loopback mechanism may be employed to synchronize transmissionsfrom the ANs 515. Example loopback mechanisms in the form of loopbacktransponders are discussed with reference to FIGS. 55A, 55B, and 55C.Referring to FIG. 63A for example, a system may have a total of 3.5 GHzof uplink bandwidth 6330 a and 3.5 GHz of downlink bandwidth 6325 aavailable. In a first switch configuration, the full 3.5 GHz uplinkbandwidth (e.g., using both of two orthogonal polarizations) may be usedconcurrently for return uplink transmissions 525 from the first usercoverage area 3460 a and forward uplink transmissions 521 from the ANarea 3450 a. Similarly, the full 3.5 GHz downlink bandwidth (e.g., usingboth of two orthogonal polarizations) may be used concurrently forforward downlink transmissions 522 to the first user coverage area 3460a and return downlink transmissions 527 to the first AN area 3450 a. Thefull uplink and downlink bandwidth may also be used in a secondswitching configuration for the second user coverage area 3460 b andsecond AN area 3450 b. While the case of two AN areas 3450 and two usercoverage areas 3460 is discussed with respect to FIG. 50B for the sakeof simplicity, any suitable number of AN areas 3450 and user coverageareas 3460 may be possible. Further, aspects discussed above withrespect to a single AN cluster (e.g., mobility, location in an aquaticbody, etc.) may be applicable to one or both of the AN clusters in thepresent example.

The above example describes AN area 3450 a as servicing anon-overlapping user coverage area 3460 a. As an alternative example, ANarea 3450 a may service user coverage area 3460 b (e.g., a user coveragearea 3460 may contain its associated AN area 3450 or some portionthereof). A similar example is generally discussed with reference toFIG. 50A in the context of a first AN area 3450 a located in NorthAmerica (e.g., which may correspond to AN area 3450 a of FIG. 50B)servicing a user coverage area 3460 a located in North America while asecond AN area 3450 b located in South America services a user coveragearea 3460 b located in South America. However, FIG. 50B shows that usercoverage areas 3460 served by different AN areas 3450 may also overlapto provide an aggregate user coverage area for a particular region. Inthis instance, the user coverage areas 3460 may be used in differenttime intervals using the switching transponders illustrated by FIGS. 51Aand 51B. Alternatively, the user coverage areas 3460 a and 3460 b may beserviced concurrently by access node areas 3450 a and 3450 b (eitherwith access node area 3450 a servicing user coverage area 3460 a whileaccess node area 3450 b services user coverage area 3460 b or withaccess node area 3450 b servicing user coverage area 3460 a while accessnode area 3450 a services user coverage area 3460 b) using the multipletransponder paths shown in FIGS. 49A and 49B. In this case, the uplinkand downlink resources used for user beams in user coverage areas 3460 aand 3460 b may be orthogonal (different frequency resources, differentpolarizations, etc.), or user beams in user coverage areas 3460 a and3460 b may use the same resources (the same frequency range andpolarization), with interference mitigated using interference mitigationtechniques such as adaptive coding and modulation (ACM), interferencecancellation, space-time coding, and the like.

As a third example, in some cases AN areas 3450 a and 3450 b combine toservice user coverage area 3460 b (or user coverage area 3460 a). Inthis case, a special loopback mechanism may not be necessary since asubset of the ANs 515 are contained within the user coverage area 3460.In some cases, the ANs 515 of AN areas 3450 a and 3450 b may beconsidered cooperating in the sense that forward uplink signals 521 fromeach of the AN areas 3450 may combine to service a single user beamcoverage area 519. Alternatively, the ANs 515 of AN area 3450 a mayservice a first subset of the user beam coverage areas 519 of usercoverage area 3460 b while the ANs 515 of AN area 3450 b may service asecond subset of the user beam coverage areas 519 of user coverage area3460 b. In some cases of this example, there may be some overlap betweenthe first and second subsets of user beam coverage areas 519 (e.g., suchthat the AN areas 3450 may be considered cooperating in some user beamcoverage areas 519 and non-cooperating in others). As a further example,AN area 3450 a may service user coverage area 3460 b at a first timeinterval (or set of time intervals) and AN area 3450 b may service usercoverage area 3460 b at a second time interval (or set of timeintervals). In some examples, the AN areas 3450 a and 3450 b maycooperate to serve user coverage area 3460 b during the first timeinterval(s) and may cooperate to serve user coverage area 3460 a duringthe second time interval(s).

In general, features of the end-to-end relay 3403 described in FIG. 41enable servicing of at least one user beam coverage area 3460 using ANs515 geographically distributed within at least one AN area 3450 that isa different physical area than the user beam coverage area 3460. In somecases, AN cluster(s) can provide high capacity to a large user coveragearea 3460. FIGS. 45A-45F, 46A, 46B, 48A, 48B, 50A, and 50B show variousexamples of such AN cluster implementations. Deploying large numbers ofANs 515 in a relatively small geographic area can provide a number ofbenefits. For example, it can be easier to ensure that more (or evenall) of the ANs 515 are deployed closer to a high-speed network (e.g.,in a region with good fiber connectivity back to the CPS 505), withinborders of a single country or region, on accessible areas, etc., withless deviation from an ideal AN 515 distribution. Implementing distinctcoverage area servicing with path selection (e.g., as in FIGS. 47A and47B) can provide additional features. For example, as described above, asingle AN cluster (and a single end-to-end relay 3403) can be used toselectively service multiple user coverage areas 3460. Similarly, asingle end-to-end relay 3403 can be used to distinguish and servicetraffic by region.

In some cases, the distinct coverage area servicing with path selectioncan enable various interference management and/or capacity managementfeatures. For example, turning back to FIGS. 48A and 48B, fourcategories of communications links can be considered: forward-linkcommunications from the AN cluster to the western active user coveragearea 3460 a (“Link A”); forward-link communications from the AN clusterto the eastern active user coverage area 3460 b (“Link B”); return-linkcommunications from the western active user coverage area 3460 a to theAN cluster (“Link C”); and return-link communications from the easternactive user coverage area 3460 b to the AN cluster (“Link D”). In afirst time interval, the eastern user coverage area 3460 b is active, sothat communications are over Link B and Link D. Because there is fulloverlap between the AN area 3450 and the eastern user coverage area 3460b, Links B and D potentially interfere. Accordingly, during the firsttime interval, Link B can be allocated a first portion of the bandwidth(e.g., 2 GHz), and Link D can be allocated a second portion of thebandwidth (e.g., 1.5 GHz). In a second time interval, the western usercoverage area 3460 a is active, so that communications are over Link Aand Link C. Because there is no overlap between the AN area 3450 and thewestern user coverage area 3460 a, Link A and Link C can use the fullbandwidth (e.g., 3.5 GHz) of the end-to-end relay 3403 during the secondtime interval. For example, during the first time interval, the forwarduplink signals 521 can be received using a first frequency range, andthe return uplink signals 525 can be received using a second frequencyrange different from the first frequency range; and during the secondtime interval, the forward uplink signals 521 and the return uplinksignals 525 can be received using a same frequency range (e.g., thefirst, second, or other frequency range). In some cases, there can befrequency reuse during both the first and second time intervals, withother interference mitigation techniques used during the first timeinterval. In some cases, the path selection timing can be selected tocompensate for such a difference in bandwidth allocation duringdifferent time intervals. For example, the first time interval can belonger than the second time interval, so that Links B and D areallocated less bandwidth for more time to at least partially compensatefor allocating Links A and C more bandwidth for a shorter time. Otheralternative frequency allocations are discussed below.

In some cases, first return uplink signals 525 are received during thefirst time interval by the plurality of cooperating user-linkconstituent receive elements 3426 a from a first portion of theplurality of user terminals 517 geographically distributed over some orall of a first user coverage area 3460 (e.g., the eastern user coveragearea 3460 b), and second return uplink signals 525 are received duringthe second time interval by the plurality of cooperating user-linkconstituent receive elements 3426 b from a second portion of theplurality of user terminals 517 geographically distributed over some orall of a second user coverage area 3460 (e.g., the western user coveragearea 3460 a). When the AN area 3450 (the AN cluster) is a subset of thefirst user coverage area 3460 b (e.g., as illustrated in FIG. 48B), theAN 515 timing can be calibrated with the end-to-end relay 3403 duringthe first time frame (e.g., when there is overlap between the usercoverage area 3460 b and the AN area 3450).

As described above, some cases can include determining a respectiverelative timing adjustment for each of the plurality of ANs 515, suchthat associated transmissions from the plurality of ANs 515 reach theend-to-end relay 3403 in synchrony (e.g., with sufficiently coordinatedtiming relative to the symbol duration, which is typically a fraction ofthe symbol duration such as 10%, 5%, 2% or other suitable value). Insuch cases, the forward uplink signals 521 are transmitted by theplurality of ANs 515 according to the respective relative timingadjustments. In some such cases, a synchronization beacon signal (e.g.,a PN signal generated by a beacon signal generator, as described above)is received by at least some of the plurality of ANs 515 from theend-to-end relay 3403, and the respective relative timing adjustmentsare determined according to the synchronization beacon signal. In othersuch cases, some or all of the ANs 515 can receive loopbacktransmissions from the end-to-end relay 3403, and the respectiverelative timing adjustments are determined according to the loopbacktransmissions. The various approaches to calibrating the ANs 515 candepend on the ability of the ANs 515 to communicate with the end-to-endrelay 3403. Accordingly, some cases can calibrate the ANs 515 onlyduring time intervals during which appropriate coverage areas areilluminated. For example, loopback transmissions via the user-linkantenna subsystem 3420 can only be used in time intervals during whichthere is some overlap between the AN area 3450 and the user coveragearea 3460 (e.g., the ANs 515 communicate over a loopback beam which canuse both a feeder-link antenna subsystem 3410 and a user-link antennasubsystem 3420 of the end-to-end relay 3403). In some cases, propercalibration can further rely on some overlap between the feeder downlinkfrequency range and the user downlink frequency range.

As discussed above, an end-to-end relay 3403 with or without selectivelycoupled transponders like those illustrated in FIGS. 49A, 49B, 51A and51B can service user terminals within a first user coverage area 3460using ANs 515 within a first AN area 3450 that is overlapping with thefirst user coverage area 3460 (e.g., both in North America), and serviceuser terminals within a second user coverage area 3460 using ANs 515within a second AN area 3450 that is overlapping with the second usercoverage area 3460 (e.g., both in South America). Alternatively, anend-to-end relay 3403 like that of FIGS. 51A and 51B can service userterminals within a first user coverage area 3460 using ANs 515 within afirst AN area 3450 that is non-overlapping with the first user coveragearea 3460 and service user terminals within a second user coverage area3460 using ANs 515 within a second AN area 3450 that is non-overlappingwith the second user coverage area 3460, as shown in FIG. 50B. As alsoshown in FIG. 50B, the first and second user coverage areas 3460 may beconfigured to at least partially overlap with each other to providecontiguous coverage to a given region (e.g., CONUS region, visible Earthcoverage region, etc.). Other similar implementations are also possible.

The system discussed with reference to FIG. 50B may, for example,include a forward beamformer 529 that generates access node-specificforward signal for each of the pluralities of ANs 515 within AN areas3450. Each of the plurality of ANs 515 within a given AN area 3450 mayobtain an access node-specific forward signal from the forwardbeamformer 529 (e.g., via a distribution network 518) during a timewindow in which the given AN cluster is active and transmit acorresponding forward uplink signal 521 to the end-to-end relay 3403.The time window in which the given AN cluster is active may include oneor more time-slices, if a time-slice beamformer architecture is employedas described above.

As described above, the system may include a means for pre-correctingthe forward uplink signals 521 to compensate for, e.g., path delays,phase shifts, etc. between the respective ANs and the end-to-end relay3403. In some cases, the pre-correction may be performed by the forwardbeamformer 529. Additionally or alternatively, the pre-correction may beperformed by the ANs 515 themselves. As an example, each of the ANs 515may transmit an access node beacon signal to end-to-end relay 3403 andreceive signaling from end-to-end relay 3403 including a relay beaconsignal and the relayed access node beacon signal (e.g., relayed fromend-to-end relay 3403). In this example, each AN 515 may adjust itsrespective forward uplink signal 521 (e.g., may adjust timing and/orphase information associated with the signal transmission) based on therelayed access node beacon signal. As an example, the AN 515 may adjustthe forward uplink signal 521 to time and phase align the relayed accessnode beacon signal with the received relay beacon signal. In some cases,the signaling described in this example (e.g., the access node beaconsignal, the relay beacon signal, and the relayed access node beaconsignal) may be received or transmitted via a feeder-link antennasubsystem 3410, as described above. Thus, in some cases, though notshown, the end-to-end relay 3403 includes a beacon signal transmitter.The beacon signal transmitter can be implemented as described above withreference to the beacon signal generator and calibration support module424 of FIG. 15 .

While portions of the above description have discussed techniques forend-to-end beamforming between a single active AN area 3450 (e.g.,selected between two or more AN areas 3450) and a single active usercoverage area 3460 (e.g., selected between two or more user coverageareas 3460), in some cases it may be desirable to have multiple distinctAN areas 3450 concurrently (e.g., cooperatively) used to provide serviceto a single user coverage area 3460. An example of such a system isdisplayed with respect to FIG. 50C, which includes AN areas 3450 a and3450 b as well as user coverage area 3460.

With reference to FIG. 50C, an example system may include multiple ANclusters (e.g., two relatively dense AN clusters). Each AN cluster maycontain multiple ANs 515 geographically distributed within therespective AN area 3450, where each AN 515 is operable to transmit arespective pre-corrected forward uplink signal 521 to the end-to-endrelay 3403. The multiple AN clusters may be used cooperatively forproviding service to user terminals 517 within the user coverage area3460. Multiple AN clusters may be employed cooperatively using a varietyof techniques. In one example, an end-to-end relay 3403 may employ afeeder-link antenna subsystem 3410 having a single feeder-link antennaelement array 3415 and a compound reflector that illuminates themultiple AN clusters.

FIG. 57 illustrates a feeder-link antenna subsystem 3410 c having asingle feeder-link antenna element array 3415 and a compound reflector5721. Each of multiple regions of the compound reflector 5721 may have afocal point 1523 (which may be the same or a different distance from thecompound reflector). A first example is illustrated in FIG. 57 in whichthe compound reflector 5721 has a single focal point (or region) 1523 a.The feeder-link antenna element array 3415 may be positioned at adefocused point of the compound reflector. As illustrated, thefeeder-link antenna element array 3415 is located inside the focal point1523 a (i.e., is closer to the compound reflector 5721 than the focalpoint 1523 a). Alternatively, the feeder-link antenna element array 3415may be located outside the focal point 1523 a (i.e., the feeder-linkantenna element array 3415 may be farther from the compound reflector5721 than the focal point 1523 a). A second example is illustrated inFIG. 57 in which the compound reflector 5721 has two focal points (orregions) 1523 b and 1523 c. In the present example, the feeder-linkantenna element array 3415 is illustrated as being located inside thefocal points 1523 b and 1523 c. Alternatively, the feeder-link antennaelement array 3415 may be located outside the focal points 1523 b and1523 c. In yet another embodiment, the feeder-link antenna element array3415 may be located inside one focal point (e.g. focal point 1523 b) andoutside another focal point (e.g., focal point 1523 c). In some cases,focal point 1523 b may be associated with a top portion of the compoundreflector 5721 while focal point 1523 c is associated with a bottomportion of the compound reflector 5721. Alternatively, focal point 1523b may be associated with a bottom portion of the compound reflector 5721while focal point 1523 c is associated with a top portion of thecompound reflector 5721. The feeder-link antenna element array 3415 mayinclude feeder-link constituent transmit elements 3419 and feeder-linkconstituent receive elements 3416, which in some cases may be the sameantenna elements (e.g., with different polarizations or frequencies usedfor transmitting and receiving, etc.).

In the transmit direction, the output of the feeder-link constituenttransmit elements 3419 may reflect from the reflector 5721 to form afirst beam group 5705 a that illuminates a first AN area 3450 (e.g., ANarea 3450 a of FIG. 50C) and a second beam group 5705 b that reflects asecond AN area 3450 (e.g., AN area 3450 b of FIG. 50C). Although notshown, in a receive direction signals from a first AN area 3450 a andfrom a second AN area 3450 b may be reflected to feeder-link constituentreceive elements 3416 of the feeder-link antenna element array 3415using compound reflector 5721.

Returning to FIG. 50C, the multiple AN areas 3450 may be usedindependently or together (e.g., cooperatively). For example, ANs ofonly one of AN areas 3450 a or 3450 b may be activated at a given time,and beamforming coefficients may be generated for forming user beamcoverage areas 519 within user coverage area 3460 from the ANs 515 ofthe active AN cluster. Alternatively, beamforming coefficients may begenerated for forming user beams within user coverage area 3460 usingboth AN clusters concurrently (e.g., cooperatively). In the forwarddirection, a forward beamformer 529 may apply the beamformingcoefficients (e.g., by a matrix product between forward beam signals anda forward beam weight matrix) to obtain a plurality of access-nodespecific forward signals for ANs 515 within both clusters to generatethe desired forward user beams. In the return direction, the returnbeamformer 531 may obtain the composite return signals from ANs 515within both clusters and apply a return beam weight matrix to form thereturn beam signals associated with the return user beams.

In some cases, AN areas 3450 a and 3450 b may be non-overlapping (e.g.,disjoint). Alternatively, AN areas 3450 a and 3450 b may be (e.g., atleast partially) overlapping. Further, at least one of AN areas 3450 aand 3450 b may be at least partially overlapping with user coverage area3460. Alternatively, at least one of the AN areas 3450 a and 3450 b maybe non-overlapping (e.g., disjoint) with user coverage area 3460. Asdiscussed above, in some cases at least one of the ANs 515 in one orboth of AN areas 3450 a or 3450 b may be disposed on a mobile platformand/or located in an aquatic body.

Referring to FIG. 50B or 50C, each of multiple AN areas 3450 may beilluminated using a separate feeder-link antenna element array 3415. Insome cases, the separate feeder-link antenna element arrays 3415 may beused concurrently (e.g., multiple AN areas 3450 may be usedcooperatively) to support service provided to a single user coveragearea 3460. With reference again to FIG. 50C, an end-to-end relay 3403may have separate feeder-link antenna element arrays 3415 illuminatingeach of AN areas 3450 a and 3450 b. In some examples, the end-to-endrelay 3403 may have separate feeder-link antenna subsystems 3410, whereeach feeder-link antenna subsystem 3410 includes a feeder-link antennaelement array 3415 and a reflector. FIG. 56A shows an end-to-end relay3403 having a feeder-link antenna subsystem 3410 a that includes a firstfeeder-link antenna element array 3415 a that illuminates the first ANarea 3450 a via a first reflector 5621 a and a second feeder-linkantenna element array 3415 b that illuminates the second AN area 3450 bvia a second reflector 5621 b. The first and second feeder-link antennaelement arrays 3415 a and 3415 b may each include feeder-linkconstituent receive elements 3416 and feeder-link constituent transmitelements 3419. FIG. 56B shows a feeder-link antenna subsystem 3410 bthat includes a first feeder-link antenna element array 3415 a and asecond feeder-link antenna element array 3415 b that illuminatecorresponding AN areas 3450 via a single reflector 5621. As illustratedin FIG. 56B, the feeder-link element arrays 3415 may be located indefocused positions in relation to the focal point 1523 of reflector5621. Although the feeder-link element arrays 3415 are displayed asbeing located beyond the focal point 1523 of reflector 5621, they mayalternatively be located closer to the reflector 5621 than the focalpoint 1523.

Similarly, multiple user coverage areas 3460 may be implemented usingseparate user-link antenna element arrays 3425 with either separatereflectors (similar to FIG. 56A) or a single reflector (similar to FIG.56B). Thus, the multiple AN areas 3450 and multiple user coverage areas3460 in FIG. 50B may be deployed using any combination of a singlefeeder-link reflector or multiple feeder-link reflectors and a singleuser-link reflector or multiple user-link reflectors. In anotherexample, a deployment similar to that shown in FIG. 50B may be achievedwith reflectors shared between feeder-links and user-links usingdifferent feeder-link and user-link frequency bands. For example, asingle antenna element array may have feeder-link constituent elementsand user-link constituent elements (e.g., in an interleaved pattern suchas that shown in FIG. 62 ). The feeder-link may use a frequency rangethat is higher (e.g., more than 1.5 or 2 times higher) to provide ahigher gain with a common reflector. In one example, the user-link mayuse a frequency range (or ranges) in the K/Ka bands (e.g., around 30GHz) while the feeder-link uses frequency range(s) in the V/W bands(e.g., around 60 GHz). Because of the narrower beamwidth at higherfrequencies, the AN area 3450 sharing the common antenna element array(and thus reflector) will be a smaller area (and concentric with) theuser coverage area. Thus, one antenna subsystem including a singleantenna element array and reflector may be used to illuminate usercoverage area 3450 a and AN area 3450 b while a second antenna subsystemincluding a single antenna element array and reflector may be used toilluminate user coverage area 3450 b and AN area 3450 a. In yet anotherexample for a deployment similar to FIG. 50B, a single antenna subsystemmay include a single reflector and two antenna element arrays as shownin FIG. 56B, where each antenna element array includes feeder-linkconstituent elements and user-link constituent elements.

Referring again to FIG. 56B, in some cases, the first feeder-linkantenna element array 3415 a may be coupled with a first subset of themultiple receive/transmit signal paths associated with the end-to-endrelay 3403 while the second feeder-link antenna element array 3415 b maybe coupled with a second subset of the multiple receive/transmit signalpaths. Thus, a first set of forward uplink signals 521 from the ANcluster having AN area 3450 a may be carried via a first subset of themultiple receive/transmit signal paths associated with the end-to-endrelay 3403. Additionally, a second set of forward uplink signals 521from the AN cluster having AN area 3450 b may be carried via a secondsubset of the multiple receive/transmit signal paths. In some cases, thefirst and second sets of forward uplink signals may both contribute toforming a forward user beam associated with at least one of the multipleforward user beam coverage areas 519 in user coverage area 3460.

FIGS. 52A and 52B show example forward and return receive/transmitsignal paths for cooperative use of multiple AN clusters, where each ANcluster is associated with a separate feeder-link antenna element array3415. Referring first to FIG. 52A, an example forward signal path 5200is shown. Forward signal path 5200 includes a first forward linktransponder 3430 e coupled between a feeder-link constituent receiveelement 3416 a of a first feeder-link antenna element array 3415 a and afirst user-link constituent transmit element 3429 of a user-link antennaelement array 3425 and a second forward link transponder 3430 e coupledbetween a feeder-link constituent receive element 3416 b of a secondfeeder-link antenna element array 3415 b and a second user-linkconstituent transmit element 3429 of the same user-link antenna elementarray 3425. An end-to-end relay 3403 may have a first set of forwardlink transponders 3420 coupled as shown by the first forward linktransponder 3430 e and a second set of forward link transponders 3430coupled as shown by the second forward link transponder 3430 e. Thus,the feeder-link constituent receive elements 3416 a of the firstfeeder-link antenna element array 3415 a may be coupled via a first setof forward link transponders 3430 e to a first subset of user-linkconstituent transmit elements 3429 of a user-link antenna element array3425 while the feeder-link constituent receive elements 3416 b of thesecond feeder-link antenna element array 3415 b may be coupled via asecond set of forward link transponders 3430 e to a second subset ofuser-link constituent transmit elements 3429 of the same user-linkantenna element array 3425. The first and second sets of user-linkconstituent transmit elements 3429 may be spatially interleaved (e.g.,alternated in rows and/or columns, etc.) within the user-link antennaelement array 3425 (e.g., as shown in FIG. 62 ).

FIG. 52B illustrates an example return signal path 5250. Return signalpath 5250 includes a first return link transponder 3440 e coupledbetween a user-link constituent receive element 3426 a of a user-linkantenna element array 3425 and a first feeder-link constituent transmitelement 3419 a of a first feeder-link antenna element array 3415 a.Return signal path 5250 also includes a second return link transponder3440 e coupled between a user-link constituent receive element 3426 b ofthe same user-link antenna element array 3425 and a second feeder-linkconstituent transmit element 3419 b of a second feeder-link antennaelement array 3415 b. An end-to-end relay 3403 may have a first set ofreturn link transponders 3440 coupled as shown by the first return linktransponder 3440 e and a second set of return link transponders 3440coupled as shown by the second return link transponder 3440 e. Thus, afirst subset of the user-link constituent receive elements 3426 a of theuser-link antenna element array 3425 may be coupled via a first set ofreturn link transponders 3440 e to feeder-link constituent transmitelement 3419 a of a first feeder-link antenna element array 3415 a whilea second subset of the user-link constituent receive elements 3426 b ofthe same user-link antenna element array 3425 may be coupled via asecond set of return link transponders 3440 e to feeder-link constituenttransmit element 3419 b of a second feeder-link antenna element array3415 b. As discussed above, the user-link constituent receive elements3426 and user-link constituent transmit elements 3429 may be the samephysical antenna elements. Similarly, the feeder-link constituentreceive elements 3416 and feeder-link constituent transmit elements 3419of a given feeder-link antenna element array 3415 may be the samephysical antenna elements.

The first and second sets of user-link constituent receive elements 3426may be spatially interleaved (e.g., alternated in rows and/or columns,etc.) within the user-link antenna element array 3425. FIG. 62 shows anexample antenna element array 6200 with spatially interleaved subsets ofconstituent antenna elements 6205. Although each constituent antennaelement 6205 is shown as a circular antenna element and the interleavedsubsets are shown as being arranged in alternating rows, the constituentantenna elements 6205 may be any shape (e.g., square, hexagonal, etc.)and arranged in any suitable pattern (e.g., alternating rows or columns,a checkerboard, etc.). Each constituent antenna element 6205 may be anexample of a user-link constituent receive element 3416 or a user-linkconstituent transmit element 3419, or both (e.g., an element used forboth transmit and receive).

With reference to FIGS. 52A and 52B where the user-link antenna elementarray 3425 is implemented as the antenna element array 6200 of FIG. 62 ,the first set of forward link transponders 3430 e may each have itsoutput coupled with one the first set of user-link antenna elements 6205a while the second set of forward link transponders 3430 e may each haveits output coupled with one of the second set of user-link antennaelements 6205 b. In addition, the first set of return link transponders3440 e may each have its input coupled with one the first set ofuser-link antenna elements 6205 a while the second set of return linktransponders 3440 e may each have its input coupled with one of thesecond set of user-link antenna elements 6205 b.

In some cases, the end-to-end relay 3403 includes a large number oftransponders, such as 512 forward-link transponders 3430 and 512return-link transponders 3440 (e.g., 1,024 transponders total). Thus,the first set of forward link transponders 3430 e of FIG. 52A mayinclude 256 transponders and the second set of forward link transponders3430 e may include 256 transponders.

In some cases, support for the use of multiple AN clusters is providedthrough characteristics of the transponders associated with theend-to-end relay 3403. Additionally or alternatively, support for theuse of multiple AN clusters may be provided using one or moreappropriately designed reflectors. Some example transponders aredescribed above (e.g., with respect to FIGS. 49A, 49B, 51A, 51B, 52A and52B), Further examples of transponder designs are discussed below. Itshould be understood that techniques described with reference to any onethe example forward link transponders 3430 and return link transponders3440 may in some cases be applicable to any other example transponder.Further, the components of the transponders may be rearranged in anysuitable fashion without deviating from the scope of the disclosure.

Only a single polarization of the receive/transmit paths (e.g., across-pole transponder) is shown in FIGS. 49A, 49B, 52A and 52B forclarity. For example, the forward-link transponder 3430 receives aforward uplink signal 521 at an uplink frequency with left-hand circularpolarization (LHCP) and outputs a forward downlink signal 522 at adownlink frequency with right-hand circular polarization (RHCP); andeach return-link transponder 3440 receives a return uplink signal 525 atthe uplink frequency with right-hand circular polarization (RHCP) andoutputs a return downlink signal 527 at the downlink frequency withleft-hand circular polarization (LHCP). In other cases, some or alltransponders can provide a dual-pole signal path pair. For example, theforward-link transponders 3430 and the return-link transponders 3440 canreceive uplink signals at the same or different uplink frequency withboth polarizations (LHCP and RHCP) and can both output downlink signalsat the same or different downlink frequency with both polarizations(RHCP and LHCP). For example, such cases can enable multiple systems tooperate in parallel using any suitable type of interference mitigationtechniques (e.g., using time division, frequency division, etc.). Insome cases, the end-to-end relay 3403 includes a large number oftransponders, such as 512 forward-link transponders 3430 and 512return-link transponders 3440 (e.g., 1,024 transponders total). Otherimplementations can include smaller numbers of transponders, such as 10,or any other suitable number. In some cases, the antenna elements areimplemented as full-duplex structures, so that each receive antennaelement shares structure with a respective transmit antenna element. Forexample, each illustrated antenna element can be implemented as two offour waveguide ports of a radiating structure adapted for bothtransmission and reception of signals. In some cases, only thefeeder-link elements, or only the user-link elements, are full duplex.Other implementations can use different types of polarization. Forexample, in some implementations, the transponders can be coupledbetween a receive antenna element and transmit antenna element of thesame polarity.

Both the example forward-link transponder 3430 and return-linktransponder 3440 can include some or all of LNAs 3705, frequencyconverters and associated filters 3710, channel amplifiers 3715, phaseshifters 3720, power amplifiers 3725 (e.g., TWTAs, SSPAs, etc.) andharmonic filters 3730. In dual-pole implementations, as shown, each polehas its own signal path with its own set of transponder components. Someimplementations can have more or fewer components. For example, thefrequency converters and associated filters 3710 can be useful in caseswhere the uplink and downlink frequencies are different. As one example,each forward-link transponder 3430 can accept an input at a firstfrequency range and can output at a second frequency range; and eachreturn-link transponder 3440 can accept an input at the first frequencyrange and can output at the second frequency band. Additionally oralternatively, each forward-link transponder 3430 can accept an input ata first frequency range and can output at a second frequency range; andeach return-link transponder 3440 can accept an input at the secondfrequency range and can output at the first frequency range.

As an example, the transponders of FIGS. 52A and 52B may be implementedin a system similar to that of FIG. 50C. In this example, some or all ofthe ANs 515 in AN area 3450 a may transmit forward uplink signals 521 incoordination with some or all of the ANs 515 in AN area 3450 b. Theforward uplink signals from the two AN clusters may thus combine toserve user terminals in user coverage area 3460. In this example, someAN clusters may affect only some user link antenna elements (e.g., someAN clusters may be associated with a subset of feeder link constituentreceive elements 3416 which may be coupled to a corresponding subset ofuser link constituent transmit elements 3429). Although the aboveexample discusses the use of two clusters, other embodiments using moreclusters are also possible.

Another example forward signal path 5300 is shown in FIG. 53A. Forwardsignal path 5300 may include some combination of LNAs 3705 a, frequencyconverters and associated filters 3710 a, channel amplifiers 3715 a,phase shifters 3720 a, power amplifiers 3725 a (e.g., TWTAs, SSPAs,etc.) and harmonic filters 3730 a. The input side of the forward-linktransponder 3430 f is selectively coupled to one of feeder-linkconstituent receive elements 3416 a or 3416 b (e.g., using a switch 4010b, or any other suitable path selection means). Each feeder-linkconstituent receive element 3416 a or 3416 b can be part of a separatefeeder-link antenna element array 3415 (e.g., each part of a separatearray 3415 of cooperating feeder-link constituent receive elements3416). The output side of the forward-link transponder 3430 f is coupledto a user-link constituent transmit element 3429 of a user-link antennaelement array 3425 (e.g., which is part of a user-link antenna elementsubsystem 3420). One or more switching controllers 4070 (not shown) canbe included in the end-to-end relay 3403 for selecting between some orall of the possible signal paths enabled by the forward-link transponder3430 f. Thus, where the example transponder 3430 b of FIG. 47A allows,for example, selective coupling between a single feeder-link constituentreceive element 3416 and multiple user-link constituent transmitelements 3429, the example transponder 3430 f of FIG. 53A allows, forexample, selective coupling between multiple feeder-link constituentreceive elements 3416 a, 3416 b and a single user-link constituenttransmit element 3429.

An example return signal path 5350 is shown in FIG. 53B. Return signalpath 5350 may include some combination of LNAs 3705 b, frequencyconverters and associated filters 3710 b, channel amplifiers 3715 b,phase shifters 3720 b, power amplifiers 3725 b (e.g., TWTAs, SSPAs,etc.) and harmonic filters 3730 b. The output side of the return-linktransponder 3440 f is selectively coupled to one of feeder-linkconstituent transmit elements 3419 a or 3419 b (e.g., using a switch4010 a, or any other suitable path selection means). Each feeder-linkconstituent transmit element 3419 a or 3419 b can be part of a separatefeeder-link antenna element array 3415 (e.g., each part of a separatearray 3415 of cooperating feeder-link constituent transmit elements3419). The input side of the return-link transponder 3440 f is coupledto a user-link constituent receive element 3426 of a user-link antennaelement array 3425 (e.g., which is part of a user-link antenna elementsubsystem 3420). One or more switching controllers 4070 (not shown) canbe included in the end-to-end relay 3403 for selecting between some orall of the possible signal paths enabled by the return-link transponder3440 f. Thus, where the example return link transponder 3440 b of FIG.47B allows, for example, selective coupling between a single feeder-linkconstituent transmit element 3419 and multiple user-link constituentreceive elements 3426, the example transponder 3440 f of FIG. 53Ballows, for example, selective coupling between a single user-linkconstituent receive element 3426 and multiple feeder-link constituenttransmit elements 3419.

As an example, the forward link transponder 3430 f of FIG. 53A may beimplemented in a system similar to that of FIG. 50C. In this example,some or all of the ANs 515 in AN area 3450 a may transmit forward uplinksignals 521 during a first time interval. Some or all of the ANs 515 inAN area 3450 b may transmit forward uplink signals 521 during a secondtime interval. Using some appropriate path selection means (e.g., aswitch), the forward link transponder 3430 f can receive input from ANarea 3450 a (e.g., via the first array of cooperating feeder-linkconstituent receive elements 3416 a) during the first time interval andfrom AN area 3450 b (e.g., via the second array of cooperatingfeeder-link constituent receive elements 3416 b) during the second timeinterval. In some such scenarios, each AN area 3450 may include a fullcomplement of ANs 515 (e.g., such that each AN area 3450 can provideappropriate beamforming over the entire user coverage area 3460).

As an example, the return-link transponder 3440 f of FIG. 53B may beimplemented in a system similar to that of FIG. 50C. In this example,some or all of the ANs 515 in AN area 3450 a may receive return downlinksignals 527 during a first time interval. Some or all of the ANs 515 inAN area 3450 b may receive return downlink signals 527 during a secondtime interval. Using some appropriate path selection means (e.g., aswitch), the return link transponder 3440 f can output to AN area 3450 a(e.g., via the first array of cooperating feeder-link constituenttransmit elements 3419 a) during the first time interval and to AN area3450 b (e.g., via the second array of cooperating feeder-linkconstituent transmit elements 3419 b) during the second time interval.In some such scenarios, each AN area 3450 may include a full complementof ANs 515 (e.g., such that the single AN area 3450 can provideappropriate beamforming over the entire user coverage area 3460).

FIGS. 54A and 54B illustrate forward and return link transponders 3430 gand 3440 g, respectively. These transponders are similar to those ofFIGS. 51A and 51B except that the components have been rearranged suchthat the switch 4010 a follows the harmonic filter(s) 3730. As discussedabove, other rearrangements of components may be possible. In somecases, this example arrangement may require fewer power amplifiers 3725and/or harmonic filters 3730. Similarly to FIGS. 51A and 51B, such anarrangement may enable selective association between AN clusters anduser coverage areas 3460. This selective association may allow flexibleallocation of capacity between two (or more) user coverage areas 3460 aswell as frequency reuse between user and feeder links (e.g., which mayincrease the capacity of the system).

As discussed above with reference to FIG. 46B, in some cases there maynot be overlap between the AN area 3450 and the user coverage area 3460,which may require the use of a separate loopback mechanism from thatdiscussed above. In some cases, the separate loopback mechanism mayinclude the use of a loopback transponder 5450, such as that shown inFIG. 55A, 55B, or 55C. In some embodiments, the loopback transponder5450 may receive AN loopback beacons (e.g., AN loopback beaconstransmitted from each AN), which may be examples of the access nodebeacon signals 2530 discussed with reference to FIG. 38 . The loopbacktransponder 5450 may retransmit the access node beacon signals 2530 andtransmit a satellite beacon (e.g., which may be generated using a relaybeacon generator 426 as described above). In some of the followingexamples, the input side of the loopback transponder 5450 is coupled toa feeder-link antenna element. Alternatively, the input side of theloopback transponder 5450 may be coupled to a loopback antenna elementthat is separate and distinct from the feeder-link antenna elementarray(s). Similarly, in some of the following examples, the output sideof the loopback transponder 5450 is coupled to a feeder-link antennaelement or a user-link antenna element. Alternatively, the output sideof the loopback transponder 5450 may be coupled to a loopback antennaelement distinct from the feeder-link antenna element array(s) and theuser-link antenna element array(s), which may the same or different thanthe loopback antenna element coupled to the input side of the loopbacktransponder 5450.

Referring to FIG. 55A, loopback transponder 5450 a may include somecombination of LNAs 3705 c, frequency converters and associated filters3710 c, channel amplifiers 3715 c, phase shifters 3720 c, poweramplifiers 3725 c (e.g., TWTAs, SSPAs, etc.) and harmonic filters 3730c. Further, as illustrated in FIG. 55B, in the case where an end-to-endrelay 3403 has multiple feeder-link antenna element arrays 3415, theinput side of the loopback transponder 5450 may be selectively coupledto one of a first feeder-link constituent receive element 3416 a of afirst feeder-link antenna element array 3415 a or a second feeder-linkconstituent receive element 3416 b of a second feeder-link antennaelement array 3415 b (e.g., using a switch 4010 b, or any other suitablepath selection means). FIG. 55A shows the output side of exampleloopback transponder 5450 a coupled to a feeder-link constituenttransmit element 3419. FIG. 55B shows the output side of exampleloopback transponder 5450 b selectively coupled (e.g., using a switch4010 a, or any other suitable path selection means) to eitherfeeder-link constituent transmit element 3419 a or feeder-linkconstituent transmit element 3419 b, which may be components of a samefeeder-link antenna element array 3415 or different feeder-link antennaelement arrays. That is, feeder-link constituent transmit element 3419 bmay be a component of the same antenna element array 3415 as feeder-linkconstituent transmit element 3419 a and/or feeder-link constituentreceive element 3416 b. As illustrated, feeder-link constituent transmitelement 3419 b is part of the same antenna element array 3415 b asfeeder-link constituent receive element 3416 b. Similarly, the inputside of loopback transponder 5450 b may be selectively coupled (e.g.,using a switch 4010 b, or any other suitable path selection means) toeither feeder-link constituent receive element 3416 a or 3416 b, whichmay be components of a same or different feeder-link antenna elementarrays 3415. The loopback transponder 5450 b of FIG. 55B may be employedin cases where the end-to-end relay 3403 supports the selective use ofone of multiple access node areas 3450 (e.g., as discussed in someexamples illustrated by FIG. 50B). Thus, switch 4010 a may be set to afirst position to provide the output of loopback transponder 5450 b tofeeder-link constituent transmit element 3419 a when a first access nodearea 3450 is active and to a second position to provide the output ofloopback transponder 5450 b to the feeder-link constituent transmitelement 3419 b when a second access node area 3450 is active. In somecases, there may be two or more feeder-link constituent transmitelements 3419, and each can be part of a separate feeder-link antennaelement array 3415 (e.g., for support of selective use of one accessnode area 3450 from two or more access node areas 3450). Referring toFIG. 55B, one or more switching controllers 4070 (not shown) can beincluded in the end-to-end relay 3403 for selecting between some or allof the possible signal paths enabled by the loopback transponder 5450 b.In some cases, a feeder-link constituent receive element 3416 and afeeder-link constituent transmit element 3419 may be associated with thesame physical structures, as described above. In some cases, the ANs 515may be able to synchronize transmissions based on a comparison of theretransmitted access node beacon signals 2530 and the satellite beacon(e.g., the transmissions from ANs 515 within one or more AN clusters maybe time and phase aligned based on the comparison).

In some cases, the feeder-link frequency range may be different from theuser-link frequency range. When the feeder-link downlink frequency rangeis non-overlapping with the user-link downlink frequency range, thetransponders that translate from the feeder-link uplink frequency rangeto the user-link downlink frequency range (e.g., using a frequencyconverter 3710) cannot be used to relay the access node beacon signals(e.g., because the ANs cannot receive and process the user-link downlinkfrequency range). In such cases, the loopback transponder 5450 may solvethe issue by translating the access node beam signals from thefeeder-link uplink frequency range to the feeder-link downlink frequencyrange. For example, feeder-link communications (e.g., forward uplinksignals 521 and return downlink signals 527) may be in a first frequencyrange (e.g., a frequency range within V/W band), and user-linkcommunications (e.g., forward downlink signals 522 and return uplinksignals 525) may be in a second frequency range (e.g., a frequency rangewithin K/Ka band). Thus, even where the AN area 3450 overlaps the usercoverage area 3460, the ANs 515 may not be able to receive AN loopbacksignals relayed via the receive/transmit signal paths (e.g., forwardtransponders 3430 and/or return transponders 3440) of the end-to-endrelay 3403.

FIG. 55C shows an example loopback transponder 5450 c that receives allAN loopback signals in the feeder-link uplink frequency range and relaysthe AN loopback signals in the feeder-link downlink frequency range.Loopback transponder 5450 c may be used in any of the above access nodecluster deployments where the access node area 3450 does not overlapwith the user coverage area 3460 (e.g., at least some of the deploymentsdiscussed with FIG. 45C, 45E, 45F, 45G or 50B). The feeder-link uplinkfrequency range and the feeder-link downlink frequency range may be partof the same band (e.g., K/Ka band, V band, etc.) or different bands. TheAN loopback signals may be received via antenna element 3455, which maybe part of a feeder-link antenna element array 3415 or may be a separateloopback antenna element. The relayed AN loopback signals may betransmitted via the same antenna element 3455 as shown, or a differentantenna element, in some cases. The loopback transponder 5450 c includesloopback frequency converter 5460, which may convert the AN loopbacksignals from one carrier frequency within the feeder-link uplinkfrequency range to a different carrier frequency within the feeder-linkdownlink frequency range. Loopback transponder 5450 c may additionallycontain one or more of LNAs 3705 c, channel amplifiers 3715 (notillustrated), phase shifters 3720 (not illustrated), power amplifiers3725 c, and harmonic filters (not illustrated).

Referring again to the example end-to-end beamforming system 3400 ofFIG. 41 , aspects of system 3400 may be modified to support cooperativeoperation of multiple AN clusters that use different frequency ranges.FIGS. 59A and 59B illustrate examples of possible geographic coverageareas for multiple access node areas 3450, each operating over adifferent frequency range, to be used cooperatively in end-to-endbeamforming for a user coverage area 3460. In the example illustrated inFIG. 59A, AN area 3450 a may be associated with Ka-band transmissionswhile AN area 3450 b may be associated with V-band transmissions. Asshown in FIG. 59A, AN areas 3450 a and 3450 b may be disjoint. In somecases, the AN area 3450 b associated with V-band transmissions may besmaller (e.g., may cover a smaller geographic area) than the AN area3450 a associated with Ka-band transmissions. In some cases AN area 3450a and AN area 3450 b may be illuminated by separate feeder-link antennaelement arrays 3415. For example, AN area 3450 a may be illuminated bythe first feeder-link antenna element array 3415 a and AN area 3450 bmay be illuminated by the second feeder-link antenna element array 3415b of the feeder-link antenna subsystem 3410 b shown in FIG. 56B. As withthe example of the first AN cluster in access node area 3450 a operatingin Ka-band while the second AN cluster in access node area 3450 b isoperating in V-band, the access node area 3450 b may be sized accordingto the difference in gain provided by the single reflector (e.g., whichmay be an example of the reflector 5621 of FIG. 56B) in the differentfrequency ranges. Alternatively, the separate feeder-link antennaelement arrays 3415 illuminating AN area 3450 a and AN area 3450 b maybe illuminated by separate reflectors (e.g., which may be examples ofreflectors 5621 discussed with reference to FIG. 56A) which may be thesame or different sizes. Alternatively, AN area 3450 a and AN area 3450b may be illuminated by the same feeder-link antenna element array 3415having multiple sets of feeder-link antenna elements 3416, 3419 with acompound reflector 5721 as shown in FIG. 57 . The different frequencyranges for different AN clusters may provide higher isolation ofdifferent subsets of feeder link elements within a single feeder-linkantenna element array, which may result in higher system capacity thanmultiple AN clusters operating in the same frequency range.

FIG. 59B illustrates an alternative arrangement of multiple AN clustersusing separate frequency ranges used cooperatively. As illustrated inFIG. 59B the two AN clusters may at least partially overlap (or one maybe completely contained within the other as shown). FIG. 59B mayillustrate examples where a single feeder-link antenna element array3415 may illuminate AN area 3450 a and AN area 3450 b (e.g.,simultaneously receive or transmit signals to both coverage areas overdifferent frequency ranges). In some cases, a given AN 515 (e.g., onelocated within AN area 3450 b) may be associated with multiple ANclusters and communicate over feeder links in multiple frequency ranges(e.g., which may be contained in different frequency bands).

FIGS. 60A and 60B illustrate example receive/transmit signal pathssupporting cooperating AN clusters operating in different frequencyranges in accordance with aspects of the present disclosure. Forwardreceive/transmit signal path 6000 of FIG. 60A includes forward-linktransponders 3430 h coupled between feeder-link constituent receiveelements 3416 a and user-link constituent transmit elements 3429 a andforward-link transponders 3430 i coupled between feeder-link constituentreceive elements 3416 b and user-link constituent transmit elements 3429b. As described above, the various user-link antenna elements may bepart of different user-link antenna element arrays 3425, which may bepositioned to provide for non-overlapping access node areas 3450 asshown in FIG. 59A or overlapping access node areas 3450 as shown in FIG.59B. Alternatively, the various user-link antenna elements may be partof the same feeder-link antenna element array 3415, in which case theaccess node areas 3450 will overlap as shown in FIG. 59B. Thefeeder-link constituent receive elements 3416 a and feeder-linkconstituent receive elements 3416 b may be interleaved within the samefeeder-link antenna element array 3415 as illustrated in FIG. 62 .

As described above, the forward-link transponder 3430 h can include someor all of LNAs 3705 a, frequency converters and associated filters 3710h, channel amplifiers 3715 a, phase shifters 3720 a, power amplifiers3725 a, and harmonic filters 3730 a. Similarly, forward-link transponder3430 i can include some or all of LNAs 3705 a, frequency converters andassociated filters 3710 i, channel amplifiers 3715 a, phase shifters3720 a, power amplifiers 3725 a, and harmonic filters 3730 a. In somecases, frequency converter 3710 h may be operable to convert signalsfrom a first feeder-link uplink frequency range to a user-link downlinkfrequency range while frequency converter 3710 i is operable to convertsignals from a second feeder-link uplink frequency range to the sameuser-link downlink frequency range.

Return receive/transmit signal path 6050 of FIG. 60B includesreturn-link transponder 3440 h coupled between a user-link constituentreceive element 3426 a and a corresponding feeder-link constituenttransmit element 3419 a and return-link transponder 3440 i coupledbetween a user-link constituent receive element 3426 b and acorresponding feeder-link constituent transmit element 3419 b. Asdescribed above, the return-link transponder 3440 h can include some orall of LNAs 3705 b, frequency converters and associated filters 3710 j,channel amplifiers 3715 b, phase shifters 3720 b, power amplifiers 3725b, and harmonic filters 3730 b. Similarly, return-link transponder 3440i can include some or all of LNAs 3705 b frequency converters andassociated filters 3710 k, channel amplifiers 3715 b, phase shifters3720 b, power amplifiers 3725 b, and harmonic filters 3730 b. In somecases, frequency converter 3710 j may be operable to convert signalsfrom a user-link uplink frequency range to a first feeder-link downlinkfrequency range (e.g., which may be the same range as the firstfeeder-link uplink frequency range described with reference to FIG. 60A)while frequency converter 3710 k is operable to convert signals from theuser-link uplink frequency range to a second feeder-link downlinkfrequency range (e.g., which may be the same range as the secondfeeder-link uplink frequency range described with reference to FIG.60A).

As described above, the various user-link antenna elements may be partof the same or different user-link antenna element arrays 3425 and thevarious feeder-link antenna elements may be part of the same ordifferent feeder-link antenna element arrays 3415. The feeder-linkconstituent transmit elements 3419 a and feeder-link constituenttransmit elements 3419 b may be interleaved within the same feeder-linkantenna element array 3415 as illustrated in FIG. 62 . Where thefrequencies supported for the feeder links by the forward-linktransponders 3430 h and 3430 i and return-link transponders 3440 h and3440 i are substantially different (e.g., one being different by morethan 1.5× from the other, etc.), the different subsets of elements 6205a, 6205 b of the antenna element array 6200 may be sized appropriatelyfor the different supported frequency ranges (e.g., constituent antennaelements 6205 b supporting a higher frequency range than constituentantenna elements 6205 a may have smaller waveguides/horns, etc.).

FIG. 64A illustrates an example frequency spectrum allocation 6400 withfour frequency ranges displayed (frequency ranges 6425 a, 6430 a, 6435a, and 6436 a). In the illustrated example, frequency ranges 6425 a and6430 a are frequency ranges within the K/Ka-bands (e.g., between 17 GHzand 40 GHz) while frequency ranges 6435 a and 6436 a are within the V/Wbands (e.g., between 40 GHz and 110 GHz). FIG. 64A may illustrateoperation of multiple AN clusters operating over different frequencyranges as shown in FIGS. 59A and 59B.

As one example, frequency spectrum allocation 6400 may be used in thescenario illustrated in FIG. 59A using an end-to-end relay 3403 havingforward and return receive/transmit signal paths 6000 and 6050 as shownin FIGS. 60A and 60B. In this example, forward uplink signals 6440 afrom AN area 3450 a may be transmitted over frequency range 6430 a(e.g., using RHCP) while forward uplink signals 6440 b from AN area 3450b may be transmitted over frequency range 6436 a (e.g., using RHCP). Thefirst set of forward uplink signals 6440 a may be received byfeeder-link constituent receive elements 3416 a while the second set offorward uplink signals 6440 b may be received by feeder-link constituentreceive elements 3416 b. For the sake of simplicity, signals may beillustrated by their span over portions or all of a frequency range(e.g., forward uplink signal 6440 a shows the frequency span of anexample of forward uplink signal 521 within frequency range 6430 a). Insome cases, a given signal may span one or more frequency ranges. Asdiscussed with reference to FIG. 60A, the two sets of forward uplinksignals 6440 are frequency converted by forward link transponders 3430 hand 3430 i (e.g., they are downconverted to the same frequency range6425 a in the Ka-band). Subsequently, the outputs of the forward-linktransponders 3430 h are transmitted by user-link constituent transmitelements 3429 a as a first set of forward downlink signals 6445 a whilethe outputs of the forward-link transponders 3430 i are transmitted byuser-link constituent transmit elements 3429 b as a second set offorward downlink signals 6445 b. In the present example, these user-linkconstituent transmit elements 3429 a, 3429 b belong to the sameuser-link antenna element array 3425 and illuminate the same usercoverage area 3460. Accordingly, the ANs 515 in access node areas 3450 aand 3450 b may be referred to as cooperating in that some fraction ofANs 515 in each area combine to serve the same user coverage area 3460.That is, at least one beamformed forward user beam providing service touser terminals 517 within the corresponding user beam coverage area 519is formed from forward uplink signals 6440 a from at least a subset ofthe ANs 515 in the first access node area 3450 a and from forward uplinksignals 6440 b from at least a subset of the ANs 515 in the secondaccess node area 3450 b.

Frequency spectrum allocation 6400 also illustrates an example offrequency allocation for return-link transmissions for the scenarioillustrated in FIG. 59A using an end-to-end relay 3403 having forwardand return receive/transmit signal paths 6000 and 6050 as shown in FIGS.60A and 60B. Return uplink signals 6450 (e.g., LHCP signals) originatingfrom user terminals 517 distributed throughout the user coverage area3460 may be transmitted over frequency range 6430 a (e.g., using LHCP)and received by user-link constituent receive elements 3426 a and 3426 bof FIG. 60B, where the user-link constituent receive elements 3426 a and3426 b belong to the same user-link antenna element array 3425. Asdescribed with reference to FIG. 60B, the return uplink signals 6450 maybe fed to return-link transponders 3440 h and 3440 i and frequencyconverted to appropriate frequency ranges 6425 a (e.g., using RHCP) and6435 a (e.g., using LHCP), respectively. The frequency converted signals6455 a and 6455 b may then be transmitted by feeder-link constituenttransmit elements 3419 a and 3419 b (e.g., which belong to separatefeeder-link antenna element arrays 3415 a and 3415 b, respectively) toANs 515 in access node areas 3450 b and 3450 a, respectively. It shouldbe understood that the frequency allocation 6400 is one example andvarious other frequency allocations may be used. For example, the returnuplink signals 6450 may be in a different frequency range (e.g., adifferent frequency range within the K/Ka band) from the forward uplinksignals 6440 a and the forward downlink signals 6445 may be in adifferent frequency range (e.g., a different frequency range within theK/Ka band) from return downlink signals 6455 a. This may, for example,allow the use of dual-pole transponders in the forward and returnreceive/transmit signal paths 6000 and 6050. Additionally oralternatively, the forward uplink signals 6440 b may be allocated withina different frequency range (e.g., a different frequency range withinthe V band) from the return downlink signals 6455 b, as illustrated.Other arrangements of the forward uplink/downlink and return/uplinkdownlink signals within the different frequency ranges may also beconsidered. For example, the return uplink signals may be allocatedwithin the same frequency range as the forward downlink signals (e.g.,using an orthogonal polarization). Additionally or alternatively, theforward uplink signals 6440 a from the ANs in the first access node area3450 a may be allocated within the same frequency range as the returndownlink signals 6455 a (e.g., using an orthogonal polarization).Coupling of forward and return receive/transmit signal paths 6000 and6050 to the various user-link and feeder-link constituenttransmit/receive elements may be selected according to the desiredfrequency range allocation.

In some examples of a single feeder-link antenna element array 3415supporting multiple AN clusters such as the multiple AN clustersillustrated in FIG. 59B, each feeder-link constituent receive element3416 and feeder-link constituent transmit element 3419 may be coupledwith multiple forward link transponders 3430. FIGS. 61A and 61Billustrate example receive/transmit signal paths supporting cooperatingAN clusters operating in different frequency ranges in accordance withaspects of the present disclosure. Forward receive/transmit signal path6100 of FIG. 61A include multiple forward-link transponders 3430 coupledbetween a feeder-link constituent receive element 3416 and multipleuser-link constituent transmit elements 3429. In some examples, afeeder-link constituent receive element 3416 receives a composite offorward uplink signals 521 from ANs 515 in multiple AN areas 3450.Following receipt by a feeder-link constituent receive element 3416, theforward uplink signals may be split (e.g., using a splitter 6005) andthe split signals may serve as inputs to forward-link transponders 3430j and 3430 k. In some examples, the splitter 6005 splits signals basedon frequency ranges (e.g., such that received forward uplink signalsoccupying a first frequency range are fed to forward-link transponder3430 j and received forward uplink signals occupying a second frequencyrange are fed to forward-link transponder 3430 k). In such a scenario,the splitter 6005 may alternatively be an example of a filter.Accordingly, frequency converters 3710 d and 3710 e may be operable toaccept inputs at different frequency ranges and output signals at thesame frequency range for superposition in the user downlink signals 522.

A return receive/transmit signal path 6150 is shown in FIG. 61B in whichreturn-link transponders 3440 couple multiple user-link constituentreceive elements 3426 a and 3426 b to a single user-link constituenttransmit element 3419. User-link constituent receive elements 3426 a and3426 b may be parts of the same user-link antenna element array 3425 orseparate user-link antenna element arrays 3425 a and 3425 b (as shown).User-link constituent receive element 3426 a may act as input toreturn-link transponder 3440 j while user-link constituent receiveelement 3426 b may act as input to return-link transponder 3440 k. Theoutputs of the return-link transponders 3440 may be fed to signalcombiner 6010 before being transmitted by feeder-link constituenttransmit element 3419 to ANs 515 in the AN areas 3450. In some cases,components of receive/transmit signal paths 6000 and 6050 may berearranged (or omitted) e.g., such that signal combiner 6010 may followharmonic filters 3430 b, splitter 6005 may precede LNAs 3705 a, etc.

FIG. 64B illustrates an example frequency spectrum allocation 6401 withfour frequency ranges displayed (frequency ranges 6425 b, 6430 b, 6435b, and 6436 b). In the illustrated example, frequency ranges 6425 b and6430 b are frequency ranges within the K/Ka-bands (e.g., between 17 GHzand 40 GHz) while frequency ranges 6435 b and 6436 b are within the V/Wbands (e.g., between 40 GHz and 110 GHz). For example, frequency ranges6425 b, 6430 b, 6435 b, and 6436 b may be the same as frequency ranges6425 a, 6430 a, 6435 a, and 6436 a illustrated in FIG. 64A. FIG. 64B mayillustrate operation of multiple AN clusters operating over differentfrequency ranges as shown in FIG. 59A or 59B.

As one example, frequency spectrum allocation 6401 may be used in thescenario illustrated in FIG. 59B using an end-to-end relay 3403 havingforward and return receive/transmit signal paths 6100 and 6150 as shownin FIGS. 61A and 61B. In this example, forward uplink signals 6440 cfrom AN area 3450 a may be transmitted over frequency range 6430 b(e.g., using RHCP) while forward uplink signals 6440 d from AN area 3450b may be transmitted over frequency range 6436 b (e.g., using RHCP). Thefirst set of forward uplink signals 6440 c may be received byfeeder-link constituent receive elements 3416 a while the second set offorward uplink signals 6440 d may be received by feeder-link constituentreceive elements 3416 b of forward receive/transmit signal paths 6100.As discussed with reference to FIG. 61A, the two sets of forward uplinksignals 6440 are frequency converted by forward link transponders 3430 jand 3430 k (e.g., they are downconverted to the same frequency range6425 b in the Ka-band). Subsequently, the outputs of the forward-linktransponders 3430 j are transmitted by user-link constituent transmitelements 3429 a as a first set of forward downlink signals 6445 c whilethe outputs of the forward-link transponders 3430 k are transmitted byuser-link constituent transmit elements 3429 b as a second set offorward downlink signals 6445 d. In the present example, these user-linkconstituent transmit elements 3429 a, 3429 b belong to the sameuser-link antenna element array 3425 and illuminate the same usercoverage area 3460. Accordingly, the ANs 515 in access node areas 3450 aand 3450 b may be referred to as cooperating in that some fraction ofANs 515 in each area combine to serve the same user coverage area 3460.That is, at least one beamformed forward user beam providing service touser terminals 517 within the corresponding user beam coverage area 519is formed from forward uplink signals 6440 c from at least a subset ofthe ANs 515 in the first access node area 3450 a and from forward uplinksignals 6440 d from at least a subset of the ANs 515 in the secondaccess node area 3450 b.

Frequency spectrum allocation 6401 also illustrates an example offrequency allocation for return-link transmissions for the scenarioillustrated in FIG. 59B using an end-to-end relay 3403 having forwardand return receive/transmit signal paths 6100 and 6150 as shown in FIGS.61A and 61B. Return uplink signals 6450 a originating from userterminals 517 distributed throughout the user coverage area 3460 may betransmitted over frequency range 6425 b (e.g., using RHCP) and receivedby user-link constituent receive elements 3426 a and 3426 b of FIG. 61B,where the user-link constituent receive elements 3426 a and 3426 bbelong to the same user-link antenna element array 3425. As describedwith reference to FIG. 61B, the return uplink signals 6450 may be fed toreturn-link transponders 3440 j and 3440 k and frequency converted toappropriate frequency ranges 6430 b (e.g., using LHCP) and 6435 b (e.g.,using LHCP), respectively. The frequency converted signals may then becombined (e.g., summed, etc.) by signal combiner 6010 and transmitted byfeeder-link constituent transmit elements 3419 to ANs 515 in access nodeareas 3450 a and 3450 b. It should be understood that the frequencyallocation 6401 is one example and various other frequency allocationsmay be used. For example, the return uplink signals 6450 a may be in adifferent frequency range (e.g., a different frequency range within theK/Ka band) than the forward downlink signals 6445 c and 6445 d.Similarly, the forward uplink signals 6440 c may be in a differentfrequency range (e.g., a different frequency range within the K/Ka band)than return downlink signals 6455 a and the forward uplink signals 6440d may be allocated within a different frequency range (e.g., a differentfrequency range within the V/W bands as illustrated) than the returndownlink signals 6455 d. This may, for example, allow the use ofdual-pole transponders in the forward and return receive/transmit signalpaths 6100 and 6150. Coupling of forward and return receive/transmitsignal paths 6100 and 6150 to the various user-link and feeder-linkconstituent transmit/receive elements may be selected according to thedesired frequency range allocation.

In some cases, the available bandwidths in a given band (e.g., K band,Ka band, etc.) for feeder-link transmissions and user-link transmissionsmay be unequal (e.g., significantly different). Additionally oralternatively, the available bandwidths for uplink and downlinktransmissions within a given band may be (e.g., significantly) unequal.As an example, a regulatory body may specify what portions of afrequency spectrum are available for various types of transmissions.

FIGS. 65A and 65B show example frequency spectrum allocations 6500 and6501 with three frequency ranges (frequency ranges 6520 a, 6525 a, and6530 a) used for the forward link and three frequency ranges (frequencyranges 6520 b, 6525 b, and 6530 b) used for the return link. In theillustrated example, frequency ranges 6520 a, 6520 b, 6525 a, and 6525 bare frequency ranges within the K/Ka-bands (e.g., between 17 GHz and 40GHz) while frequency ranges 6530 a and 6530 b are within the V/W bands(e.g., between 40 GHz and 110 GHz). FIGS. 65A and 65B may illustrateoperation of multiple AN clusters operating over different frequencyranges as shown in FIG. 59A or 59B.

Referring to FIG. 65A, forward uplink signals 6540 a from AN area 3450 amay be transmitted over frequency range 6525 a (e.g., using RHCP) whileforward uplink signals 6540 b from AN area 3450 b may be transmittedover frequency range 6530 a (e.g., using RHCP). As discussed withreference to FIG. 60A or 61A, the two sets of forward uplink signals6540 are frequency converted by forward link transponders 3430 to thefrequency range 6520 a. In the example illustrated in FIG. 65A, thecombined bandwidth of frequency ranges 6525 a and 6530 a equals thebandwidth of frequency range 6520 a. Thus, forward uplink signals 6540 aare frequency converted (e.g., via frequency converters in the forwardlink transponders of forward receive/transmit signal paths 6000 or 6100)to forward downlink signals 6545 spanning a first portion 6521 a offrequency range 6520 a while forward uplink signals 6540 b are frequencyconverted (e.g., via frequency converters in the forward linktransponders of forward receive/transmit signal paths 6000 or 6100) toforward downlink signals 6545 spanning a second portion 6521 b offrequency range 6520 a. A given beamformed user beam in the usercoverage area 3460 may span all of frequency range 6520 a, in which casethe user beam is formed from both forward uplink signals 6540 a and 6540b. Where each user beam formed by forward downlink signals 6545 uses asubset of frequency range 6520 a, some user beams may be formed by firstportion 6521 a of frequency range 6520 a and some user beams may beformed by second portion 6521 b of frequency range 6520 a. Additionallyor alternatively, in some cases some user beams may be formed bycooperative superposition of forward downlink signals 6545 associatedwith frequency range 6521 a and forward downlink signals 6545 associatedwith frequency range 6521 b (e.g., frequency ranges 6521 a and 6521 bmay partially overlap to enable cooperatively forming user beams in usercoverage area 3460 with forward uplink signals 6540 from different ANclusters). In another example, one or both of frequency ranges 6525 a or6530 a may have the same bandwidth as frequency range 6520 a (e.g., orthe combined bandwidth of frequency ranges 6525 a and 6530 a may exceedthe bandwidth of frequency range 6520 a), and thus up to all forwarduser beams may be formed by cooperative superposition of forwarddownlink signals associated with frequency ranges 6521 a and 6521 b.

FIG. 65B shows example return link allocations where at least one accessnode area 3450 utilizes frequency ranges within a different band than isused for the user coverage area 3460. Specifically, the user terminals517 may transmit return uplink signals 6550 over a frequency range 6520b (e.g., within K/Ka bands), which may be received via two sets ofuser-link constituent receive elements 3416 as shown in either FIG. 60Bor 61B, and frequency converted (e.g., via frequency converters in thereturn link transponders 3440 of return receive/transmit signal paths6050 or 6150) to a first set of return downlink signals 6555 a infrequency range 6525 b and a second set of return downlink signals 6555b in frequency range 6530 b. The first and second sets of returndownlink signals 6555 a, 6555 b may be transmitted from the samefeeder-link constituent transmit element 3419 (as shown in FIG. 61B), orfrom different feeder-link constituent transmit elements 3419 (as shownin FIG. 60B). As with FIG. 65A, the combined bandwidths of frequencyranges 6525 b and 6530 b are illustrated to be equal to the bandwidth offrequency range 6520 b. Thus, a first portion 6560 a of return uplinksignals 6550 may be frequency converted and transmitted by a first setof return link transponders 3440 as return downlink signals 6555 a whilea second portion 6560 b (which may or may not overlap with the firstportion 6560 a) may be frequency converted and transmitted by a secondset of return link transponders 3440 as return downlink signals 6555 b.Thus, some return user beams may be formed by performing return linkbeamforming processing on portions of return downlink signals 6555 a andsome return user beams may be formed by performing return linkbeamforming processing on portions of return downlink signals 6555 b.Additionally or alternatively, some return user beams may be formed byperforming return link beamforming processing on portions of returndownlink signals 6555 a and return downlink signals 6555 b (e.g., someportions of return downlink signals 6555 a and 6555 b may cooperate toform a single return user beam). In some cases, one or both of frequencyranges 6525 b or 6530 b may have the same bandwidth as frequency range6520 b (e.g., or the combined bandwidth of frequency ranges 6525 b and6530 b may exceed the bandwidth of frequency range 6520 b), and thus upto all return user beams may be formed by cooperative superposition ofreturn downlink signals 6555 a and 6555 b.

FIGS. 66A and 66B illustrate example receive/transmit signal pathssupporting cooperating AN clusters operating in different frequencyranges in accordance with aspects of the present disclosure. Forwardreceive/transmit signal path 6600 of FIG. 66A includes forward-linktransponders 3430 l coupled between feeder-link constituent receiveelements 3416 a and user-link constituent transmit elements 3429 andforward-link transponders 3430 m coupled between feeder-link constituentreceive elements 3416 b and user-link constituent transmit elements3429. As described above, the forward-link transponder 3430 l caninclude some or all of LNAs 3705 a, frequency converters and associatedfilters 37101, channel amplifiers 3715 a, phase shifters 3720 a, poweramplifiers 3725 a, and harmonic filters 3730 a. Similarly, forward-linktransponder 3430 m can include some or all of LNAs 3705 a, frequencyconverters and associated filters 3710 m, channel amplifiers 3715 a,phase shifters 3720 a, power amplifiers 3725 a, and harmonic filters3730 a. In some cases, frequency converter 3710 l may be operable toconvert signals from a first feeder-link uplink frequency range (e.g.,frequency range 6525 a of FIG. 65A) to a first portion of a user-linkdownlink frequency range (e.g., frequency range 6521 a of FIG. 65A)while frequency converter 3710 m is operable to convert signals from asecond feeder-link uplink frequency range (e.g., frequency range 6530 aof FIG. 65A) to a second portion of the same user-link downlinkfrequency range (e.g., frequency range 6521 b of FIG. 65A). Theforward-link transponders 3430 couple multiple feeder-link constituentreceive elements 3416 a and 3416 b to a single user-link constituenttransmit element 3429. Feeder-link constituent receive elements 3416 aand 3416 b may be parts of the same feeder-link antenna element array3415 or separate feeder-link antenna element arrays 3415 a and 3415 b(as shown). Feeder-link constituent receive element 3416 a may act asinput to forward-link transponder 3430 l while feeder-link constituentreceive element 3416 b may act as input to forward-link transponder 3430m. The outputs of the forward-link transponders 3430 may be fed tosignal combiner 6610 before being transmitted by user-link constituenttransmit element 3429 to user terminals 517 in the user coverage areas3460. In some cases, components of receive/transmit signal paths 6600and 6650 may be rearranged (or omitted) e.g., such that signal combiner6610 may follow harmonic filters 3430 b, splitter 6605 may precede LNAs3705 a, etc.

Return receive/transmit signal path 6650 of FIG. 66B includesreturn-link transponder 3440 l coupled between a user-link constituentreceive element 3426 and a corresponding feeder-link constituenttransmit element 3419 a and return-link transponder 3440 m coupledbetween a user-link constituent receive element 3426 and a correspondingfeeder-link constituent transmit element 3419 b. As described above, thereturn-link transponder 3440 l can include some or all of LNAs 3705 b,frequency converters and associated filters 3710 n, channel amplifiers3715 b, phase shifters 3720 b, power amplifiers 3725 b, and harmonicfilters 3730 b. Similarly, return-link transponder 3440 m can includesome or all of LNAs 3705 b frequency converters and associated filters3710 o, channel amplifiers 3715 b, phase shifters 3720 b, poweramplifiers 3725 b, and harmonic filters 3730 b. In some cases, frequencyconverter 3710 n may be operable to convert signals from a first portionof a user-link uplink frequency range (e.g., frequency range 6560 a ofFIG. 65B) to a first feeder-link downlink frequency range (e.g.,frequency range 6525 b of FIG. 65B, which may be the same range as thefirst feeder-link uplink frequency range described with reference toFIG. 66A) while frequency converter 3710 o is operable to convertsignals from a second portion of the user-link uplink frequency range(e.g., frequency range 6560 b of FIG. 65B) to a second feeder-linkdownlink frequency range (e.g., frequency range 6530 b of FIG. 65B,which may be the same range as the second feeder-link uplink frequencyrange described with reference to FIG. 66A). Following receipt by auser-link constituent receive element 3426, the return uplink signalsmay be split (e.g., using a splitter 6605) and the split signals mayserve as inputs to return-link transponders 3440 l and 3440 m. In someexamples, the splitter 6605 splits signals based on frequency ranges(e.g., such that received return uplink signals occupying a firstfrequency range are fed to forward-link transponder 3430 l and receivedreturn uplink signals occupying a second frequency range are fed toforward-link transponder 3430 m). In such a scenario, the splitter 6605may be an example of one or more filters. Accordingly, frequencyconverters 3710 n and 3710 o may be operable to accept inputs atdifferent frequency ranges or portions of a frequency range and outputsignals in different frequency ranges in feeder downlink signals 522.

As described above, the various feeder-link antenna elements may be partof the same or different feeder-link antenna element arrays 3415. Thefeeder-link constituent transmit elements 3419 a and feeder-linkconstituent transmit elements 3419 b may be interleaved within the samefeeder-link antenna element array 3415 as illustrated in FIG. 62 . Wherethe frequencies supported for the feeder links by the forward-linktransponders 3430 l and 3430 m and return-link transponders 3440 l and3440 m are substantially different (e.g., one being different by morethan 1.5× from the other, etc.), the different subsets of elements 6205a, 6205 b of the antenna element array 6200 may be sized appropriatelyfor the different supported frequency ranges (e.g., constituent antennaelements 6205 b supporting a higher frequency range than constituentantenna elements 6205 a may have smaller waveguides/horns, etc.).

Access Nodes Supporting Multiple Independent Feeder Link Signals

In some examples, one or more ANs 515 may support multiple feeder links(e.g., transmission of multiple forward uplink signals and/or receptionof multiple return downlink signals). In some cases, ANs 515 supportingmultiple feeder links may be used to reduce the number of ANs. Forexample, instead of having M ANs 515 where each AN 515 supports onefeeder link, the system may have M/2 ANs 515, where each AN 515 supportstwo feeder links. While having M/2 ANs 515 reduces spatial diversity ofthe ANs 515, signals between the ANs 515 and the end-to-end relay atdifferent frequencies will experience different channels, which alsoresults in channel diversity between the two feeder links. Each AN 515may receive multiple access node-specific forward signals 516, whereeach access node-specific forward signal 516 is weighted according tobeamforming coefficients that are determined based on a channel matrixassociated with the corresponding transmit frequency range. Thus, whereeach AN 515 supports two feeder links, each AN 515 may be provided afirst access node-specific forward signal determined based in part on afirst forward uplink channel matrix for forward uplink channels betweenthe ANs 515 and the end-to-end relay 3403 over a first frequency rangeand a second access node-specific forward signal determined based inpart on a second forward uplink channel matrix for the forward uplinkchannels between the ANs 515 and the end-to-end relay 3403 over a secondfrequency range. Similarly, on the return link, each AN 515 may obtain afirst composite return signal based on a first return downlink signal ina third frequency range (which may be the same frequency range or in thesame band as the first frequency range) and a second composite returnsignal based on a second return downlink signal in a fourth frequencyrange (which may be the same frequency range or in the same band as thesecond frequency range). Each AN 515 may provide the respective firstand second composite return signals to the return beamformer 513, whichmay apply beamforming coefficients to the first composite return signalsdetermined based in part on a first return downlink channel matrix forthe return downlink channels between the end-to-end relay 3403 and theANs 515 over the third frequency range and apply beamformingcoefficients to the second composite return signals determined based inpart on a second return downlink channel matrix for the return downlinkchannels over the fourth frequency range.

Systems employing M/2 ANs 515 may have reduced system capacity whencompared to having M ANs 515, but the system cost reduction (e.g.,including set up and maintenance costs) may be substantial while stillproviding acceptable performance. Additionally, a number of ANs 515other than M/2 may be used, such as 0.75-M, which may provide similar orgreater performance at reduced cost when compared to M ANs 515 eachsupporting only one feeder link. Generally, where M ANs 515 would beused each supporting a single feeder link (e.g., a single feeder uplinkfrequency range and a single feeder downlink frequency range), X-M ANs515 may be used where each AN 515 supports multiple feeder links, whereX is in the range of 0.5 to 1.0.

Returning to FIGS. 45A and 45B, the X-M ANs 515 may be distributedwithin the access node area 3450 and may service user terminals 517within user coverage area 3460 via beamformed user beams, where one ormore user beams are beamformed using multiple feeder link signals fromat least one AN 515. The multiple feeder links may be supported via asingle set of feeder-link constituent antenna elements (e.g., a singlefeeder-link antenna element array 3415), or separate feeder-linkconstituent antenna elements (separate feeder-link antenna elementarrays 3415 for each feeder link).

A single feeder-link antenna element array 3415 and a single reflectormay be used to support multiple feeder links for each AN 515 usingeither the forward or return receive/transmit signal paths 6000, 6050 ofFIGS. 60A and 60B (e.g., separate subsets of feeder-link constituentantenna elements within the same feeder-link antenna element array3415), or the forward and return receive/transmit signal paths 6100,6150 of FIGS. 61A and 61B (e.g., splitters and combiners used tomultiplex the multiple feeder links using the same set of feeder-linkconstituent antenna elements). Where the difference in frequency rangesbetween the multiple feeder links is substantial (which may be desirableto increase channel diversity), the dimensions of the access node area3450 may depend on the higher frequency feeder link. For example, wherea first feeder link is supported in a frequency range around 30 GHzwhile a second feeder link is supported in a frequency range around 60GHz, the access node area is limited to the area illuminated by thesingle feeder-link antenna element array 3415 via the single reflector.Thus, some path diversity for the lower frequency range may be lost.Alternatively, a first feeder-link antenna element array 3415 a may beused to support a first frequency range while a second feeder-linkantenna element array 3415 b is used to support a second frequencyrange. In this case, separate reflectors may be used, and may be sizedappropriately to provide coverage of a same access node area 3450 at thedifferent frequencies. For example, where a first feeder link issupported by a first feeder-link antenna element array 3415 a and afirst reflector in a frequency range around 30 GHz while a second feederlink is supported by a second feeder-link antenna element array 3415 band a second reflector in a frequency range around 60 GHz, the firstreflector may be larger (e.g., having twice the reflector area) than thesecond reflector to account for the difference in antenna gain at thedifferent frequencies.

Frequency allocation for the different feeder links may be performed invarious ways including that shown in FIG. 64A, 64B, 65A, or 65B. Thatis, a first feeder link may use carrier frequencies within frequencyranges 6425 a and 6430 a (e.g., in K/Ka bands) while a second feederlink uses frequency range 6435 a (e.g., in V/W bands) as shown in FIG.64A. Alternatively, the first feeder link may use carrier frequencieswithin frequency ranges 6430 b (e.g., in K/Ka bands) while a secondfeeder link uses frequency range 6435 b (e.g., in V/W bands) as shown inFIG. 64B. In yet another alternative, the first and second feeder linksmay both use frequencies different from the user links as shown in FIGS.65A and 65B where a first feeder link uses frequency ranges 6525 a and6525 b (e.g., in V/W bands) while a second feeder link uses frequencyranges 6530 a and 6530 b (e.g., in V/W bands). In some examples, thefirst feeder link and second feeder link may use frequency ranges thatare substantially different (e.g., the lowest frequency in one frequencyrange may be greater than 1.5 or 2 times the lowest frequency in theother frequency range). As discussed above, the bandwidth for eachfeeder link frequency range may be less than the bandwidth for the userlink frequency range, or one or more of the feeder link frequency rangesmay have the same bandwidth as the user link frequency range. In somecases, the correlation of the signals associated with the first andsecond feeder links may be inversely proportional to the bandwidthseparation between the two signals (e.g., such that two signals whosefrequency ranges are adjacent within the Ka-band are more correlatedthan a Ka-band signal and a V-band signal or two signals withnon-adjacent frequency ranges within the Ka-band). This effect is aresult of the signals with adjacent frequency ranges experiencingsimilar atmospheric effects, whereas signals with a greater degree ofbandwidth separation will experience different atmospheric effects,which contributes to the induced multipath.

CONCLUSION

Although the disclosed method and apparatus is described above in termsof various examples, cases and implementations, it will be understoodthat the particular features, aspects, and functionality described inone or more of the individual examples can be applied to other examples.Thus, the breadth and scope of the claimed invention is not to belimited by any of the examples provided above but is rather defined bythe claims.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, are to be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” is usedto mean “including, without limitation” or the like; the term “example”is used to provide examples of instances of the item in discussion, notan exhaustive or limiting list thereof; the terms “a” or “an” mean “atleast one,” “one or more” or the like.

Throughout the specification, the term “couple” or “coupled” is used torefer broadly to either physical or electrical (including wireless)connection between components. In some cases, a first component may becoupled to a second component through an intermediate third componentdisposed between the first and second component. For example, componentsmay be coupled through direct connections, impedance matching networks,amplifiers, attenuators, filters, direct current blocks, alternatingcurrent blocks, etc.

A group of items linked with the conjunction “and” means that not eachand every one of those items is required to be present in the grouping,but rather includes all or any subset of all unless expressly statedotherwise. Similarly, a group of items linked with the conjunction “or”does not require mutual exclusivity among that group, but ratherincludes all or any subset of all unless expressly stated otherwise.Furthermore, although items, elements, or components of the disclosedmethod and apparatus may be described or claimed in the singular, theplural is contemplated to be within the scope thereof unless limitationto the singular is explicitly stated.

The presence of broadening words and phrases such as “one or more,” “atleast,” or other like phrases in some instances does not mean that thenarrower case is intended or required in instances where such broadeningphrases may be absent. Additionally, the terms “multiple” and“plurality” may be used synonymously herein.

While reference signs may be included in the claims, these are providedfor the sole function of making the claims easier to understand, and theinclusion (or omission) of reference signs is not to be seen as limitingthe extent of the matter protected by the claims.

What is claimed is:
 1. A method of operation by a satellitecommunications system for forming forward user beams, for serving userterminals in corresponding forward user beam coverage areas, the methodcomprising: transmitting channel sounding probes from a plurality ofgeographically distributed access nodes, wherein an associated satelliteincludes a plurality of cooperating transmit signal paths that eachrelays a unique superposition of the channel sounding probes; receivingamplitude and phase feedback from each designated terminal among aplurality of designated user terminals, the amplitude and phase feedbackfrom each designated user terminal based on reception by the designateduser terminal of the channel sounding probes as relayed via theplurality of cooperating transmit signal paths and indicative of anend-to-end forward channel between each access node and the designateduser terminal; calculating forward beam weights for weighting aplurality of forward uplink signals synchronously transmitted from theplurality of access nodes, wherein the forward beam weights arecalculated in dependence on end-to-end forward gains estimated from theamplitude and phase feedback, so that superpositions of the forwarduplink signals relayed by the satellite form a plurality of forward userbeams, each forward user beam centered on a respective one among theplurality of designated user terminals; and synchronously transmittingthe plurality of forward uplink signals from the plurality of accessnodes according to the forward beam weights.
 2. The method according toclaim 1, further comprising maintaining the forward beam weights on anongoing basis for ongoing transmission of the plurality of forwarduplink signals, based on repeatedly transmitting channel soundingprobes, receiving corresponding amplitude and phase feedback, andrecalculating the forward beam weights.
 3. The method according to claim1, wherein each access node transmits a respective one among theplurality of forward uplink signals based on receiving a respectiveaccess-node specific forward signal from a Central Processing System(CPS) in a ground network of the satellite communications system, andwherein synchronously transmitting the plurality of forward uplinksignals from the plurality of access nodes comprises the CPS performingdelay equalization by sending the respective access-node specificforward signals at respective time-domain offsets, the respectivetime-domain offsets accounting for path delay differences among theplurality of access nodes.
 4. The method according to claim 1, whereineach access node transmits a respective one among the plurality offorward uplink signals based on receiving a respective access-nodespecific forward signal from a Central Processing System (CPS) in aground network of the satellite communications system, and whereinsynchronously transmitting the plurality of forward uplink signals fromthe plurality of access nodes comprises the CPS performing delayequalization by sending respective time-domain offsets to be applied bycorresponding ones among the plurality of access nodes for transmissionof the respective forward uplink signals, the respective time-domainoffsets accounting for path delay differences among the plurality ofaccess nodes.
 5. The method according to claim 1, wherein synchronouslytransmitting the plurality of forward uplink signals from the pluralityof access nodes comprises each access node determining a transmissiontiming for the forward uplink signal transmitted by the access nodebased on the access node based on aligning a beacon signal transmittedby the access node with a corresponding relay beacon signal transmittedby the satellite.
 6. The method according to claim 5, wherein theforward uplink signal transmitted by each access node corresponds to arespective access-node specific forward signal received from a CentralProcessing System (CPS) of the satellite communications system, theaccess-node specific forward signal containing time stamps indicatingdesired signal arrival times at the satellite, and wherein the accessnode determines the transmission timing for the forward uplink signaltransmitted by the access node based on aligning signal samplescontained in the access-node specific forward signal with the beaconsignal according to the time stamps.
 7. The method according to claim 1,wherein calculating the forward beam weights for weighting the pluralityof forward uplink signals comprises generating a M×K forward beam weightmatrix, where M is the number of access nodes in the plurality of accessnodes and K is the number of forward user beams in the plurality offorward user beams, such that each column of the M×K forward beam weightmatrix corresponds to one of the K forward user beams and comprises avector of M access-node specific weights calculated such thatsimultaneous transmission of a signal from the M access nodes accordingto the vector of M access-node specific weights results in the pluralityof cooperating transmit signal paths of the satellite relayingsuperpositions of the signal that form the corresponding one among the Kforward user beams.
 8. The method according to claim 7, whereinsynchronously transmitting the plurality of forward uplink signals fromthe plurality of access nodes according to the forward beam weightscomprises: splitting each forward beam signal among K forward beamsignals into a corresponding set of M duplicate signals, such that thereare K sets of M duplicate signals, and wherein each one of the K forwardbeam signals corresponds to a respective one among the K forward userbeams and carries data for user terminals served by the respectiveforward user beam; for each one of the K sets of M duplicate signals,forming a corresponding set of weighted signals by weighting eachduplicate signal in the set by the corresponding element from thecorresponding column of the M×K forward beam weight matrix; forming Maccess-node specific signals by, for each one of the M access nodes,combining the weighted signals across the K sets of M weighted signalsthat correspond to the access node; and distributing the M access-nodespecific signals to the M access nodes.
 9. The method according to claim1, wherein the end-to-end forward gains estimated from the amplitude andphase feedback account for forward gains from the plurality of accessnodes to the plurality of cooperating signal paths onboard thesatellite, forward gains of the cooperating signal paths, and forwardgains from the plurality of cooperating signal paths to the designatedterminals.
 10. A satellite communications system configured to formforward user beams, for serving user terminals in corresponding forwarduser beam coverage areas, the satellite communications systemcomprising: a ground network comprising a Central Processing System(CPS) communicatively coupled to a plurality of geographicallydistributed access nodes; a satellite comprising a plurality ofcooperating transmit signal paths; and wherein the ground network isconfigured to: transmit channel sounding probes via the plurality ofaccess nodes, wherein each one among the plurality of cooperatingtransmit signal paths onboard the satellite relays a uniquesuperposition of the channel sounding probes; receive amplitude andphase feedback via the plurality of access nodes, respective amplitudeand phase feedback received from each designated terminal among aplurality of designated user terminals, the amplitude and phase feedbackreceived from each designated user terminal based on reception by thedesignated user terminal of the channel sounding probes as relayed viathe plurality of cooperating transmit signal paths and indicative of anend-to-end forward channel between each access node and the designateduser terminal; calculate forward beam weights for weighting a pluralityof forward uplink signals synchronously transmitted from the pluralityof access nodes, wherein the forward beam weights are calculated independence on end-to-end forward gains estimated from the amplitude andphase feedback, so that superpositions of the forward uplink signalsrelayed by the satellite form a plurality of forward user beams, eachforward user beam centered on a respective one among the plurality ofdesignated user terminals; and synchronously transmit the plurality offorward uplink signals from the plurality of access nodes according tothe forward beam weights.
 11. The satellite communications systemaccording to claim 10, wherein the CPS includes circuitry that isconfigured to maintain the forward beam weights on an ongoing basis forongoing transmission of the plurality of forward uplink signals, basedon repeatedly transmitting channel sounding probes, receivingcorresponding amplitude and phase feedback, and recalculating theforward beam weights.
 12. The satellite communications system accordingto claim 10, wherein each access node is configured to transmit arespective one among the plurality of forward uplink signals based onreceiving a respective access-node specific forward signal from the CPS,and wherein, for synchronously transmitting the plurality of forwarduplink signals from the plurality of access nodes, the CPS includescircuitry configured to perform delay equalization, based on sending therespective access-node specific forward signals at respectivetime-domain offsets, the respective time-domain offsets accounting forpath delay differences among the plurality of access nodes.
 13. Thesatellite communications system according to claim 10, wherein eachaccess node is configured to transmit a respective one among theplurality of forward uplink signals based on receiving a respectiveaccess-node specific forward signal from the CPS, and wherein, forsynchronously transmitting the plurality of forward uplink signals fromthe plurality of access nodes, the CPS includes circuitry configured toperform delay equalization, based on sending respective time-domainoffsets to be applied by corresponding ones among the plurality ofaccess nodes for transmission of the respective forward uplink signals,the respective time-domain offsets accounting for path delay differencesamong the plurality of access nodes.
 14. The satellite communicationssystem according to claim 10, wherein, for synchronously transmittingthe plurality of forward uplink signals from the plurality of accessnodes, each access node includes circuitry configured to determine atransmission timing for the forward uplink signal transmitted by theaccess node, based on aligning a beacon signal transmitted by the accessnode with a corresponding relay beacon signal transmitted by thesatellite.
 15. The satellite communications system according to claim14, wherein the forward uplink signal transmitted by each access nodecorresponds to a respective access-node specific forward signal receivedfrom the CPS, the access-node specific forward signal containing timestamps indicating desired signal arrival times at the satellite, andwherein the access node is configured to determine the transmissiontiming for the forward uplink signal transmitted by the access nodebased on aligning signal samples contained in the access-node specificforward signal with the beacon signal, according to the time stamps. 16.The satellite communications system according to claim 10, wherein theCPS includes circuitry configured to calculate the forward beam weightsas a M×K forward beam weight matrix, where M is the number of accessnodes in the plurality of access nodes and K is the number of forwarduser beams in the plurality of forward user beams, such that each columnof the M×K forward beam weight matrix corresponds to one of the Kforward user beams and comprises a vector of M access-node specificweights calculated such that simultaneous transmission of a signal fromthe M access nodes according to the vector of access-node specificweights results in the plurality of cooperating transmit signal paths ofthe satellite relaying superpositions of the signal that form thecorresponding one of the K forward user beams.
 17. The satellitecommunications system according to claim 16, wherein, for synchronouslytransmitting the plurality of forward uplink signals from the pluralityof access nodes according to the forward beam weights, the CPS includescircuitry configured to: split each forward beam signal among K forwardbeam signals into a corresponding set of M duplicate signals, such thatthere are K sets of M duplicate signals, and wherein each one of the Kforward beam signals corresponds to a respective one among the K forwarduser beams and carries data for user terminals served by the respectiveforward user beam; for each one of the K sets of M duplicate signals,form a corresponding set of weighted signals by weighting each duplicatesignal in the set by a corresponding element from the correspondingcolumn of the M×K forward beam weight matrix; form M access-nodespecific signals by, for each one of the M access nodes, combining theweighted signals across the K sets of M weighted signals that correspondto the access node; and distribute the M access-node specific signals tothe M access nodes.
 18. The satellite communications system according toclaim 10, wherein the end-to-end forward gains estimated from theamplitude and phase feedback account for forward gains from theplurality of access nodes to the plurality of cooperating signal pathsonboard the satellite, forward gains of the cooperating signal paths,and forward gains from the plurality of cooperating signal paths to thedesignated terminals.